Mystery in the park 04-17-21

There aren't very many organized events in the park behind our house.  Used to be the Scouts, girl or boy, would do adventures, but not this past year.  And some years ago there was a summer "camp" where the kids would put up tents, play games, and have lunch, only to take everything down in the late afternoon. This is the second time this group has set up back there and I haven't figured out what they are doing.  My guess would be "war games", but surely not with such young children.  There are no weapons in sight.  I see some plastic tubs beside some of the structures, which appear to be filled with air.  There doesn't appear to be any organized activity, just kids running around and arraying themselves near the structures.  About 3pm they had packed up their equipment and left.  Any ideas?

I cannot make out what the lettering says. 

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