N.S.
I would think with all of the fun things you already have in the yard - you do not need to stress out over trying to plan, coordinate and oversee a planned project. Let the kids play and have fun - that's what parties are for.
I personally hate the newer traditions of not opening gifts at the party. As kids get a little older then what to see the sparkle and appreciation their friend has for the special gift they selected. I see diappoint in their little faces when they find out the gifts won't be opened until later.