Panel 1: We start off with Robin digging up the yard next to the fence and shoveling dirt into a pail. Hmmmm.... This won't end well.
Panel 2: He packs it down. This is made easy because the soil is somewhat sandy.
Panel 3: Having done that, he dumps it back out.
Panel 4: Having gathered clumps of it in both hands, he twirls around in place letting it swirl around him.
Panel 5: Having put some in the the of a toy truck, he lays down in it and pretends to drive the thing.
Panel 6: He then picks up the truck and dumps it out.
Panel 7: Next, he leans back into the fence and kicks up the dirt.
Panel 8: He then changes position as as to finally coat himself evenly in topsoil.
Panel 9: Deanna then motions for him to come inside.
Panel 10: As she washes him, she wonders (with a rueful smirk) how little sand could get so many places. First, it's dirt, not sand. Second, she's scrubbing him so hard she's getting water in his ears. Third, she, as stillayuppie said, obviously did something stupid and tracked his dirt into the house. Fourth, she could have avoided this and, as stillayuppie said, bought a sandbox. At least she's not gobsmacked, though.
Summary: It's another example of a woefully unprepared parent confused by her child's normal, everyday behavior. Lynn must have been made of fail as a Mom.