Birthday Party For A Six Year Old

1. I would like the parents of the six year old guests to remain at the birthday party instead of just "dropping" them off for 3 hours. ( We have an inground pool and will also have pony rides and the parents need to supervise.) How do I word this on the invitation?

2. When people RSVP they always ask what they can get for "Susie" or even worse they ask my husband what to buy? As I do not know how much this person wishes to spend I feel uncomfortable suggesting a gift. I was told it is acceptable to register at a store for your child`s birthday party gifts. Is this correct? And if so, do I mention on the invitation "Susie is registered at this store?