S.D.
I run a home daycare. I think it is totally fine for your dd to invite a few kids, or all of them, whatever she (and you) want to do. Sometimes the bday child invites only the same gender, or only similar age, or only the children he/she plays with most. I don't think there have ever been hurt feelings, nor has anyone felt obligated to bring a gift. I would put on there, like someone else said, presents are not required. I held a large bday party for my dd's 1st bday (she is now 9) and put "Your presents are not required, only your presence." Most people brought something, but a lot did not, and that was totally fine. I think, being a park, any age child would do great, it will be a lot of fun, and it gives the parents at the daycare a chance to get to know each other better. Enjoy!