Yes, they can. Parent jumpers will be charged the standard weekday or weekend rates ($15 or $19 respectively). They will also have to purchase grip socks at $3 if they do not already have their own.
The host parents should indicate on the booking form whether they would like the costs of these additional jumpers to be charged to them, or to the respective parents. The event coordinator will keep track of and clarify the total number of jumpers at the end of the party, before payment of the balance amount.