From timharrod, who asks : It looks on the surface like everyone’s missing an item, and the reason turns out to be that Wallace built a raft out of them, but it’s still opaque: why would a kid building a sand castle be looking for a rope? Why the lifeguard? Is Wallace actually on a raft or is that Mom’s hypothesis?
Comics I Don't Understand 
I don’t think anyone is missing an item. The pictures just refer to what the characters are thinking or talking about. That’s the way it has worked in the past with this strip.
I think the rope is missing from a ball and cup game.
https://museum.wales/traditional_toys/cup_and_ball/
Maybe the kid with the missing flag wanted to build a sandcastle, and put the flag on top. What is he holding? Seashells, maybe?
The Lifeguard is whistling at Wallace, because he’s too far out;
Amelia might have been planning a bonfire (she likes dangerous stuff);
Rose (holding the shovel that goes with the pail at her feet) is very safety-conscious, and has noticed that Amelia is not wearing the recommended ankle rope for her surfboard;
Sterling (holding a complete fish skeleton) may have wanted the flag to mark its grave;
Spud is worried about his good friend Wallace; and
Mom is planning to skin him alive for pulling such a stupid stunt.