Nope, would give that a hard swerve.
The likelihood is that you will be guilted, attempts to strong arm you and it could be the final nail in your relationship.
These are users with two salaries that couldn't care less about you and your children.
I bet you would have loved a months holiday stateside?
Free childcare for 3 children for the summer holidays must run in a couple of thousand.....
People like this are users and will be outraged at you daring to not want to be free au pair for them.
Free childcare is far more important than any relationship with you.
If you go, you will be kicking yourself.
They could well send the children over to you on the day, asking to visit, to put you on the spot.
Think about your marriage and how you will feel about your husband if his family use a gathering to gang up on you?
Talk to your husband...don't go.
You need space from these users.