The more the merrier! At the very least another pair of eyes to watch kids is always helpful. And with a lot of the kids, being around an adult who is approachable but not authoritative to the extreme is probably very helpful, depending on the kid's background!
Haha, Leto would just smile at Nemesis and try to be very modest. She doesn't do much, just what she'd want someone to do for her own children. But Nemesis should come to the parties! If anyone can convince old rich men to give to charity, she can! She can think of it as checking up on them?
She says there will be free food and wine, so he should come! I think she tries, subconsciously, to keep kids away from gods associated with death, so she might not invite him to participate with the actual kids. Unless he can get her a discount on trips for groups of kids on the Staten Island Ferry!