To think this dad is being very rude and inconsiderate?

I recommend have them come along. You or your husband escort the birthday boy and friend while the rest of you do whatever with the younger kids. Maybe trade off here and there. Maybe dad doesn’t feel comfortable having his young son away from him and decided to tag along.

You have to be honest and tell Marks father your son has invited his best friend to His birthday treat. You thought about that chat after the school pickup and have realised it's not what your son wants so sorry . Or say no space for three adults and four children in a car is not legal in a 4(5) seat car. As two passengers would not be in a seatbelt. So sorry it's no happening

Ooh just say no that's it wouldn't give explanation it's your sons birthday so I'd simply say no sorry it's not what my son wants and it's a little odd you've asked to come along your son offer of course still stands and that's that there's not even a doubt that I'd even lose sleep over this situation put ye foot down

Just go together but make it clear you're leaving them to their own devices once you're there

Not read all the thread, so sorry if this has already been asked, but has your car got adequate car seats to accommodate these extra children? That could be your get out clause if it hasn’t.

I would let them come, but I would say obviously you can't fit in our car and maybe bring a friend for younger child to go on the rides with, so the older children can have some quality time together on sons Birthday.

Have you considered that Mark asked his Dad to come along. I don't know if there is a mum in the mix as she hasn't been mentioned but if there isn't, and noone else for childcare, Dad would need to bring his other son along too.

Maybe you could suggest that the 6 year old brings his own friend to go on rides with and hang out with so that the party boy can do his own thing. With his friend.

I doubt mobile phone coverage would be awful if he messages his own child😂

If I was in a theme park with my younger child I would just message my older one direct and not go through mum.

Just be honest with the dad. could you explain to dad that your son would like his birthday party just to be him and his friend as the dynamics are different when the younger brother is there.

Does the younger brother really want to be the unwanted third wheel?

I really don't think it's fair on him. He'd surely have more fun doing something else with his Dad.

No she can't. It's a public theme park.

Don't be ridiculous! Of course the OP can tell the kid that he can't come, it's her trip and her DS's birthday treat. Also, the younger child is far more likely to accept it from a relative stranger, without having a melt down, than if his Dad tells him he can't go.

Even at those you suggested I would have thought 8 year olds would stay with parents. I certainly wouldn’t let mine wander off, I’d let them go on rides alone but I’d be stood waiting for them

No! My husband always used to take the kids and their mates to theme parks at all ages - we had a Merlin pass - and no-one has ever said we'll come too. Changes the dynamic.

Is he assuming it is your car or suggesting you all go in their car if they have a large people carrier?

At 8 years old I would be nervous of my child going over an hour away to a theme park with another family unless they were a close family which I was close to and trusted a lot, it's possible they are suggesting they want to come along as they don't want to suggest they don't trust you but don't otherwise feel comfortable with their son going.

Also with the car is it possible his parents are nervous of your driving and therefore trying to suggest you all go in their car?

I'm on the fence - I would be reluctant to allow my child to go a theme park on their own with another family. I think the is possibly being rude but I think it better the dad attends to be honest.

Thinking about this more I imagine that the reason why he’s suggested coming like this is because he’s not actually comfortable with letting his DS go off to a theme park over an hour away with virtual strangers. He’s suggested you all share a car because the alternative is for him to say that on balance he’d rather his DS not go. It would cause far more disappointment to Mark if he was now told he’s not able to go than if his dad decides to tag along “for the benefit of his younger brother.”

TBH OP I think you’ve lost this one. I think if you say no to the car (which is perfectly understandable, although it’s possible they have a car big enough in which case he’ll offer to drive) then he’ll either say that Mark can’t go or he’ll say that he’ll drive his family, including Mark, and meet you there.

He’s actually trying to save the day IMO rather than ruin it.

Is it a possibility that 'Mark' doesn't feel comfortable going on his own with another family and so the dad thinks he is solving the problem by tagging along?
Also I've had a situation a few times with a certain parent in my sons class where she would never take us up on our offers of taking him to and from a birthday related activity. She would make excuse after excuse as to why she had to come along aswell or meet us there and it turns out that she and her husband were not comfortable with other people driving their children around and supervising them. Fair enough but it would have been so much easier if they'd just said that from the start instead of coming up with random excuses each time! It made me dread inviting this boy to anything because quite frankly I didn't want to have to deal with the whole family at my sons birthday.
I'm not really surely what to suggest but if it were me I wouldn't want the parent and sibling tagging along, it would spoil the whole dynamic of the day.

Or maybe he is not happy for eldest child to go off with relative strangers to a theme park with all the risks etc and doesn't have anyone to look after DS2 - Maybe eldest has never been to a theme park and he wants to be there for the experience - there are a whole load of reasons - I think I would say OK BUT not share a car etc

If you have a 5 seater cat then it's an easy refusal?
Yes, it's cheeky to invite yourself like that in front of the children.
Otherwise just say no. That doesn't work for us.

I would have left your 2 yo out of it entirely. Make it a "boys day" and send the 2 dads and the 2 boys if the other dad is keen to come.
The 7 seater thing is a bit odd to me as almost everyone I know has a 7 seater ( mind the back 2 seats are only suitable for goblins or fairies as they are teensy. Then there's the middle row for 3 ( and then mum and dad in the front )That's 7?

Please never give advice to anyone, ever!

I think it is better this way. If Mark is bringing his younger sibling and you have 2, then there will be an even number of children and more adults to keep an eye on everyone. A theme park is a public place, you can't stop anyone else going there.