Apologies in advance. I'm tired. This might be a ramble.
Mostly I give them whatever they need and set the routine aside. But I kinda always have a plan in the back of my mind to get us back on track when they get well. So for example I always go to them when they cry in the night but I almost never bring them out of the room. If i took my son out and cuddled on the couch in front of the tv I would have him screaming to watch tv for weeks after. I always give pain relief, water and a pat if that's all they need but I pick them up and rock and cuddle also if needed. My kids are both REALLY different. My older DS would try to take that and run with it after he got well. DD not so much. So for DS I kinda just only give him what he needs and no more iykwim. If I really felt that he needed me with him all night I would sleep on a mattress on his bedroom floor. I wouldn't bring him into my bed. I'm trying to tell him that I will always meet his needs (physical and emotional) but there are still rules and boundaries at bed time. Some other things I do to help get back on track. Tell him that now he is feeling well that I expect him to sleep all night. I find that telling my kids verbally what I expect from them always helps. I give pain relief before bed for a few nights after sickness symptoms have eased. I make sure the room temp is constant so they dont wake up from being hot/cold. I give them a leakproof water cup in their cot because I find they are super thirsty after illness.
I also mostly stick to their normal daytime sleep schedule and don't let them sleep all day.
With all this stuff I have found they they slip back into good nighttime sleeping without having to let them cry. Good luck. X