To want DH to consult me before making childcare plans

is this for valentines specifically or something else-if it is just a random day ask him to change it

OK before having read this I was thinking about the Mil- I'm close to my Mil so I would call her and ask her if it's not going to be too much for her and check in if she would like to switch day for my son.
But since you don't care for your Mil and it's just about your son's routine I think you are being a little precious. I don't mean that in a nasty way. I have been guilty of having been a bit precious with my firstborn as well. With number two you get much more relaxed- you kind of have to😅 which is a good thing for everyone involved. Your son can feel that your nervous on drop off and might be kicking off because of it. As if you go into leaving him feeling confident that Mil can deal with whatever comes up you will enjoy yourself more and your son will feel better about his sleep over as well.

Yes Yabu.

Or suggest they put him in pjs before leaving the cousins house as he will most probably fall asleep in the car on the way home - if he is a routine boy

can't you just say to DH to swap nights out, says its so lovely of you to organise etc but you don't want to stress MIL. So it doesn't clash for MIL and make her exhausted looking after 2 toddlers - when she can actually enjoy looking after both separately

TBH I think looking after 2 toddlers is a lot for MIL regardless

or he could just check with his mum?!

This is how women end up shouldering all the parenting and burning out. Because they want to control everything, they can't loosen up and want everything done exactly as they want all the time.

It might unsettle him but it's for one night. He will recover.

Is it less that you think your DH should run things by you than that you think DH should be aware of and consider all the permutations?

Can you suggest to him now that he changes the plan with MIL to another night?

Surely your MIL would have said no if she felt she wasn't up for it?
My own MIL would usually have my own DD and her older cousin for a sleepover at the same time as they like to play together. She likes having them together. If she's fit and not usually a person afraid to say no, assume she knows what she's doing.

Second point, I understand wanting your DH to have maybe said something (I know myself and DH would mention it if we're organising childcare) however he was trying to do something nice here. If you bring it up, you may put him off making the effort again.

Don't overthink it, and enjoy your childfree night out

This really is a none issue. So he’ll be more tired than usual for a day?

It's one night...I think sometimes in the desperate need to get sleep some people forget that life goes on and sometimes one late night really isn't that big of a deal and I say that as someone who has a child who didn't sleep for any more than a few hours at a time for years.

So? Once in a while doesn't matter. You can't be rigid and expect routines never to change. Your husband is doing a nice thing by arranging a night out.

Does it matter for one day? I used to be like you and I look back and think they soon would have got back into their routine.

I can remember my Mum offering to have my brothers 2 children as well as my 2 when I'd asked her ( well in advance) to have mine overnight for my first big night out since DC1 was born. I was really cross, i knew she'd struggle with 2 the first time, that's why I hadn't asked her till my youngest was capable of sleeping through. She couldn't manage all of them, had a terrible night, the DC all kept each other up, and delivered all 4 of them back to my house at 8am shouting at me. There was no real reason why she needed to have my brother's children as well as mine, she just thought it might be fun. There was nothing organised, not even beds, we did it all when we arrived, and she had no plan.
It went so wrong she never had any of them again overnight.
My children are not horrors, they were usually pretty well behaved, and like a lot of children thrived on routine.
OP I totally get why you are annoyed. You'd want your DC to have a nice quiet time, with usual routines in place in a nice quiet atmosphere, not having granny racing around trying to deal with the differing needs after keeping DC up well after their bedtime. It's obviously a recipe for disaster.
Tell Dh it's too much for MiL, and another day would work better. Explain that you don't want it all to go tits up, and then MiL not wanting to babysit again, and then rearrange it with Mil yourself.

I would give my right arm to have a parent or in-law who could occasionally take my child.

MIL could have said no if it was too much for her. Or as others have said, it could suit her better to get all the babysitting done on one day.

Enjoy your childfree time and stop fretting.

Id you ask him to consult you, you’ll be putting yourself at the top of the list for people-arranging now and forever more. There are four adults here: you, DH, MIL and SIL. Your permission isn’t required, the other three can sort it out amongst themselves. If you feel bad or embarrassed that MIL is going to feel knackered, just ask your DH if he knew this or discussed it with his mum before he made the plan and suggest picking another day. You don’t need to be the one in charge of everyone and everything - be careful what you wish for!

Surely this is between your DH and MIL? She should have said no if she's already committed elsewhere.

Like others have said though she may well prefer having one very busy night than doing multiple nights babysitting.

So you do all the arranging of your child, expect for the times your husband does. Then when he does you don't like that he has and suppose everyone (MILs and SILs in this case) else's feeling on it.

That's a "I do everything" martyrdom in it's infancy. Not too late to stop i growing, if you accept he can arrange things too.

It’s not up to you to manage your MIL’s commitments. She should be able to say no to her own son, and your SIL can take her concerns up with her own brother.

2 issues here - one is should he have checked other is the plan itself.

Should he have checked - yes, maybe, that would be good communication. BUT that's outweighed by it being nice for him to be proactive. I'd love my husband to proactively make a plan to take me out AND organize childcare. That would be the dream. (NB one time he took me away for a surprise weekend and literally the day before realized he hadn't organized childcare because that's usually my domain. Lol).

Other thing is the plan itself - you just need to chill. The plan is fine. Your child spending time with his cousin is a nice thing. Your MIL will be fine. I say this from a place of kindness as I was similarly routine-obsessed with my eldest and in hindsight I should have chilled out, life would have been more fun.

Enjoy your date night!

You are making problems. Your MIL is a grown woman surely capable of deciding her own limits.

When kids where smaller my working friends complained there husband never sorted out child care. One husband got it in the neck when he called her to ask where their child was so he could pick them up 🤣.

the fact this seems a regular clash suggests that the MIL would rather babysit once than twice or that the SIL monopolises the babysitter