@Toria33
TBH, at this point I'd be telling H (not so 'D') that neither you nor DC will be anywhere near his parents. And as I said above, they should NOT have keys to your house. In fact, they shouldn't be allowed in your house because they can't be trusted, but TBH I don't know how you'd be able to stop H from opening the door to them or how quickly you could make an exit with DC if he did.
I will say that it is your responsibility to protect yourself and DC. You know you can't trust your iLs so you need to be pro-active. If your iLs come in close to you or DC for a hug or kiss, put out your arms with hands palms up towards them (a 'stop!' posture) or push them away and say loudly "No, I can't trust you not to have had nuts recently". You do have that power, so use it! Don't stand there passively. Have DC in your arms (not H's) ready to say 'no'.
I have Coeliac so obvs I'm not dealing with potential anaphylaxis. Someone kissing my cheek after eating gluten will not make me ill so it's easily handled by just not eating anything prepared by someone I feel is 'untrustworthy'. But if one of my children had developed Coeliac, I'd be damned if I'd allow ANY person I deem untrustworthy around that child if I felt there was the least chance they'd give that child anything with gluten in it.
Although I can and do take care of myself, I would seriously consider divorce if my H didn't take the protection of our child seriously. And I'd try to get a child agreement prohibiting him from having our child near his parents (probably nearly impossible, but I'd still try so there was an official record).