Maybe there’s a bigger dynamic here: mom and dad are used to making demands, and you are used to going along with these demands despite your misgivings.
In the past, minimizing your reactions, gaslighting you about the legitimacy of your concerns, and ploughing on ahead regardless of your stated objections was the way it was, and now that the stakes are higher, all of you feel weird that you seem to mean it this time.
Hold your ground. The safety of two little people depend on it, and quite truthfully, let’s talk about your PSYCHOLOGICAL safety.
Don’t leave your precious children with two people who don’t respect you, and who are not TRUSTWORTHY.
Trustworthy people respect boundaries and would be knocking themselves out to do everything possible to keep their grandbabies safe, and their daughter satisfied that her children were entrusted to the most capable, caring, attentive, loving, safety conscious people possible, NOT two careless drunks who hoped to multitask — By also socializing by the wide open pool and pond, putzing around in the garage while the toddler played in a nearby puddle of battery acid and with a junkyard dog, or was taking walks with the “tickling uncle” who always came around and just ‘couldn’t get enough of the kids.’
I truthfully wonder why they want to get ahold of your kids so bad? It doesn’t seem like they want to do anything special with them at all. It seems like they just want to showboat them around to other people without you there, until they get sick of them, and then they’ll shove them in an upstairs bedroom for a so-called nap, which is…sketchy and weird.