If they keep them together as a group, they could always have extra services that day so that if regular worshippers show up and find the place packed, they can leave and come back to a later service or can plan to attend a later service in the first place. Many churches already do this for high services such as Christmas and Easter.
In our Catholic church, the first communions are scheduled -- "regular worshipers" know when the first communions will be, so they won't just show up and find the place packed -- they'll make plans accordingly to be considerate and choose a different mass that day or take their chances on finding a seat. It's not a matter of being "excluded" but of a special mass for those kids and family members, and the congregation at large helping to ensure that those closest to the children are able to share that special event with them.
I think on May Day the girls wear their first communion dresses so the congregation at large can ooh and ahh. But it's been a few years so I can't recall.