K.P.
This is a very tough question to answer. How many people are attending? Will everyone be going to the church first or is the party on a different day or much later in the day?
My suggestion for a First Communion party for a young child... do something at your home if you can. You can have it catered and rent tables/chairs/tent whatever you need, but the odds are pretty good that after being "dressed up" for HOURS, your daughter or son will be anxious to get into play clothes and run around. My parents always did ours at their house and invited people immediately following the service (but bring a change of clothing).
If that's not an option, do a brunch following the Mass at a child-friendly restaurant with a private room. Bring a change of clothes for your child.