My kids both had hand foot and mouth. With both of them, doctors cleared them to go back to daycare when they had been fever free for 24 hours and didn't feel sick. We didn't have to wait until the blisters were gone. Kids are most contagious before the blisters and fever even show up. It may seem horrible that the kid was at the party, but if he had the fever a couple of days before he had probably been cleared by a doctor.
My daughter had it worse than my son. But she only missed one day of daycare because the fever showed up on Friday. By Monday she was fine. She was 15 months and had little blisters on her hands and feet. I never saw them in her mouth but I think there must have been because she quit nursing cold turkey. The worst was that she had the blisters in her diaper area and even the tiniest bit of wetness was really irritating and painful. Friday and Saturday were horrible!
My son was a little older and had blisters on his hands and feet but they never bothered him. He had a few in his mouth and didn't eat much for a couple of days. He only felt sick the couple of hours when he had the fever. He missed just a day of daycare and was never really bothered by the blisters.
Search Hand Foot Mouth on mayo.edu. It's the only medical website I trust. Mayo is where we get our primary care and I know the info on their website is trustworthy.