So, they now know that the gift isn't timed well, will cost you a small fortune, make the pregnant or new mommies uncomfortable, but they want to spend time with just the adults... because it is what they want for Christmas. It sounds like this gift is for them, and you enjoying the theatre is only a coincidence.
You may need to just say, thanks, but no thanks and accept that they aren't interested in putting the money towards something else.
Your dh needs to tell his parents that the gift would be wonderful in about x-months or years, but it isn't going to happen now.
Please try not to let them upset you.