My mum’s big birthday and daughter wants to do a gymnastics show

Team daughter here. Its an excellent compromise and I don't see a problem.

I’d let your daughter do the show. She’s been practising for weeks, it’s not like she’s just joined and it’s a spur of the moment thing plus it’s her last primary show! Let DH bring her up after and do something nice on the Sunday together.

My DH is keen not to upset my daughter and to stay with her to do the show and drive her up late afterwards. They’d get to my parents’ at 9ish - well after the meal is finished. My DH says I need to prioritise DD

He's right. & for all you know your Mum may be understanding of this, even if you're not.

This. We had similar with one of my stepchildren and an in law kept laying it on thick how their birthday was important. Ooh that got the DSCs back up and they snapped and said the competition was important to them. I was so proud of them standing up

Could you stay an extra day so that your parents can see DD and DH for a bit longer even if they miss the actual meal? Young children generally don't care about nice meals anyway so she probably wouldn't engage with your mum that much during the meal anyway, nor DH if they don't get on.

This

The question is, how upset will your mother be if DD misses part of the celebrations including the meal, if DD is going to see her later in the day? We all know that children don't really get a lot out of these big family occasions unless they are able to be unruly with their cousins, so it's not like DD will miss out and I'm fairly sure the birthday girl will be mostly chatting with adults rather than getting intimately involved in the children's shenanigans. I suspect you will be the one most put out - can you put that aside for your DD and maybe have a girlie day out with Granny some other time?

Personally family comes first, time isnt always a given and things can change quickly especially in regards to health, I think it's really important to celebrate these milestones with loved ones and make memories. This family event has been organised for a long time with lots of people travelling a distance to attend. The show was a later addition. If it was a regional competition then perhaps different but for a show? Grandma's 70th birthday trumps that.

You started with rather categorical "Family always comes first", which is hardly compatible with "I don't ever recall an occasion where they've ever said no to anything"...

Anyway, let's leave this part aside :)

The OP knew that the dates will come relatively late, so it comes with some commitment of "trying to make it work". Who else should exercise a little flexibility than the girl's own family? Especially when she actually can do both- visit the grandma on her birthday and have her show.

It sounded that the OP's main concern was what her brother would say, which just sounds to me like the least important voice in the whole situation, but something which might resonate with "family always comes first" approach.

Again, with respect, your family couldn't sound any more different to ours. I'm sorry you had that experience but it's not ours.
I could repeatedly tell you I don't ever recall an occasion where they've ever said no to anything (apart from 2 Birthday parties when we were actually abroad on holiday) but you seem to be convinced you know us better, which is honestly a bit odd. Why not just accept different people do things differently.
Yes there was a commitment to the team but 2 wks notice means they'd already made other plans. I don't suppose they could be expected to put life on hold until that date was announced.

I would go with your DH suggestion. Let her do the show and come up to birthday afterwards.

My mother would say the same as you are saying. She is convicted it was our choice. And we were meeting friends, just in a way she planned, no space for more spontaneous/last minute plans, because the grandparents were planned in advance.

Anyway, what about the commitment to her dance team?

Your DH’s proposal sounds very sensible.

They will both be there later, and If your DH doesn’t have an easy relationship with your mum, it’s not clear why his presence is so necessary for the whole thing.

I would absolutely let my daughter do the show (and have done in the past with family occasions and dance shows)

The girl has also made a commitment to her gymnastics team mates. Does that not count?

Op knew there was a risk the dates would end up clashing before the DD started practicing for the show.

It's not like it's a 'better.offer' to another party. It's a show that has probably had hours of work and practice put into it.

My children adored their grandparents (who lived 3 hours away) and still go to stay with their widowed grandmother (now in her 90s) because they want to. We are a close family and big on celebrating landmark events. But my parents would never have made a thing about their grandchildren being at a meal in these circumstances, especially if they were going to be there the rest of the weekend. Perhaps that is why my DCs adored them - they were not demanding in anyway, just endlessly loving and confident in the knowledge that they were loved back.
As I have already posted, I would not be aggrieved if one of my DGCs did not attend my 70th birthday meal later this year in these circumstances, especially if I was going to see them the rest of the weekend.
I understand btw that the OP says the meal cannot be rearranged because they have paid a deposit but, if 100% attendance is so important to the grandmother, it would be worth speaking to the restaurant about switching to lunch the next day instead. As long as the restaurant is still getting the custom, they may be happy to be flexible if given notice.

This

But that's absolutely not even remotely similar to our family/life. As I said my children would most definitely choose their grandparents. They both do lots of hobbies, they both have days out every weekend, they both see friends all the time, we actually have big friends days out every school holiday and weekend trips away with them.

The commitment is the pre booked meal, not seeing family, sorry I thought that bit was obvious.

Why don't you just host the dinner at yours, problem solved

Re: honouring the prior commitment - the gymnastics show was the first, even if the date was annoyingly non-specific. But also it wasn't the granddaughter who double-booked things! The 11yo was not in charge of checking whether a restaurant dinner clashed with her end-of-term show and presumably no one ran the dates by her. She shouldn't be made to honour a commitment that she didn't actually make just because the adults dropped the ball.

I was the child who was often unable to join friends over the weekend because we had an existing plan to visit grandparents. While I liked them, the "commitment" was souring it a lot.
Grandparents shouldn't be a "commitment", family should be an organic part of life.

Isn't training with a team a kind of commitment as well? And, even when the date is unclear, commitment to "really try to make it work" and in this case, coming bit later on Saturday would make both work.

Talking as a grandparent, I would never want one of my grandchildren to miss an event like this to come for a meal out for my birthday.
Your dd has been practicing for weeks for this event and it’s just unfortunate the dates clash. I’m assuming, as it is a show, then there are choreographed routines that removing one child from is going to cause untold hassle for coaches and team mates. You should never have signed your dd up for an event that may have clashed if it was so important for her to be at the dinner.

Family always comes first here but we're very close. My children would definitely choose to see their Grandparents over anything, they adore them.
Also it's a commitment that was already made, you don't cancel something you've already arranged because a perceived 'better offer' comes along.

I completely agree with this

Is she the only child? It might actually be a more relaxed lunch without the only child and the son in law the birthday girl dislikes