K.P.
People do assume that siblings are invited, which is kind of strange to me because I don't recall that being the trend when we were kids!
For my son's party, we addressed the invite to the child and then included a little note:
"Jumpin Jakes is great fun for children under the age of 6! John can't wait to celebrate with his classmates!"
I was really pleased to see that the folks at the front desk enforced this too. We had a couple of guests show up with older siblings (without asking) and the girl at the desk let them know that the older child needed to stay with the adults because the play equipment was not age-appropriate. We paid for a package party that included 15 children and the place asked us for a guest list. If the name wasn't on the guest list, the receptionist asked the parent for $8 the non-invited child to "play". We were left out of the whole thing and it was great!
As for what to spend on a gift, it depends on the child. If it's a "good friend" we do a $10 item and a $15 TRU gift card. If it's a classmate, we'll do the $15 item and include the gift receipt!