The parents will be cringing at this idea of theirs in years to come.
At a gess, they don't know much about children's parties as thier dd is so young.
Assume they expect all parents to stay (as you would with 2 year olds) and the fiver is towards food and drink for the adults.
If you suspect they are just plain odd and mean, ignore the invite and don't go.
If you think they are not, talk to them about the party plans and entrance fee before you make a decision. At least that way they will get an inkling that their invite is and unusual one.
I can't help thinking how it could be on the day - all those invites sent out but no guests, their dd all ready for her party, parents wondering what they did wrong