OP, everyone here is right.
Do not let your SIL and your MIL hold your children, and the fact that THEIR birthday is special, hostage.
Seriously.
Ignore them....
DO NOT ENGAGE....
Your comments about 'the fallout from MIL', and the fact that your husband very passive-aggressively sits back, and gives them things THEIR way, speaks volumes.
This would not even be a topic of conversation with me.
It would not be enabled.
I would make my plans, issue the invites WAY in advance... and however it works out from there is just the way it is.
I would celebrate ON their birthday with any family who chooses to care about your children and celebrate their birthday.
And, as the kids are older, their birthday would not really be so much about your husbands family, and only them... and it would be all about YOUR KIDS AND THEIR FRIENDS AND WHAT THEY WANT.
I feel for you.
I really do.
BTDT with the whole 'everything always HAS to be all about and please my husbands mom and or dad.
Am even dealing with this right now with my son's best friend, who, as an almost grown teenager, usually can't make or commit to plans with my son, 'because grandma this... grandma that..."
Very sad.
At this point, your children are involved.
If your husband doesn't want to stand with you and support you and his own children when it comes to their birthday. Well, as has been said here on the DIS, it isn't usually an inlaw problem, it is a marriage problem.
I have to wonder as I am sitting here writing... if birthdays, and probably other occasions, are all about 'FAMILY' celebrations with HIS family. How do you celebrate your own birthday with your husband.. How about your anniversary... How about holidays.... I do think I know the answer to this one.