This is tricky - there's a lot going on in all the different relationships, isn't there.
I can understand that you'd love them to take both children for a bit, but I can also understand that that feels full on to them. My ILs (the nearby grandparents) used to have one child at a time - they liked to really focus on giving that one a nice time and would have found them all (I had 3 under 2 at one point!) hard to manage together.
Have they actually taken your little girl for the day/a sleepover? Because if they haven't yet, isn't there the option of just letting them and next time there's an offer, suggesting that they do something with your son instead (perhaps not a sleepover unless everyone is happy - he still seems pretty young for that to me).
It's a tricky balance - you want things to be fair, but you don't want to deny your kids opportunities (or yourself, if having one does feel easier to you). I'm super-alert to any signs of grandparental favouritism (we get a fair bit of it, mostly fairly minor - e.g. Grandad has watched masses of my nephews' sporting fixtures, but has only been to one of my son's and none of my daughter's; they tend to take the cousins on more elaborate/higher value days out that my kids would also love but don't get offered), but ultimately the grandparents do work hard at and value their relationships with all the grandchildren (and us!) and so I try hard to see the big picture (and have a good moan about it with DH when necessary).
Finally, as you've said, at 7 months pregnant with two little ones to care for, you're probably not feeling at your most rational and resilient. Tread lightly for now, if you can.