Please try to stop focusing on what you think your in-laws should do. A lot of prople would not think that a lunchtime dinner out is anywhere as celebratory as an evening one, and I hope no-one wants to derail this by making that into a discussion.
@TinkerBeee I am not going to say that
"you have a husband problem rather than an in-law one"
But this is something that you and your DH need to, and can solve. Either he doesn't go at all, because it doesn't fit in with your family's routine - imo he should definitely not go to the dinner without you, so if you have no-one to babysit both of your DC (in exactly the same way that you and your DH do it) then neither of you should go - or, if you can afford it, you all stay near to the venue (or even at it, if it is in a hotel), and you all go to the dinner, hoping that one night won't disrupt your routine too much (I wouldn't have suggested this in the days when smoking was still allowed inside public buildings).
I think that if you were to do my second suggestion, then you both should just assume that you are all invited to the dinner, and let your in-laws know what is happening. If they then tell you (either of you) that they don't want young children there, then you should both decline the invitation completely. Your husband may be hurt by his parents attitude, but I would be very hurt and upset if my husband had gone to anything at my young children's bedtime if it took both of us to put them to bed, and there were no suitable alternatives.
As parents, most of us have to make many sacrifices, especially when our DC are young, some of them may only be relatively small ones, as in your/your husbands potential case here, and some may be huge sacrifices. This is something that everyone who decides to have a child needs to realise - along, of course, with many other things as well - so hopefully your DH won't be too put out if his parents say no, and he therefore has to stay at home.