The prank is Kendall's idea. Their father is sitting peacefully under the canopy, looking relaxed for once. Kendall whispers, "Wouldn't it be funny if…" and gestures at the water pistols they've been playing with, then at Logan.
"Yeah!" says Shiv. "We all spray him together."
This does sound like fun. Roman only hesitates for a second, remembering the time he jumped out at Logan in his study, but then he thinks that if all three of them go in at the same time Logan won't be angry at Roman in particular.
Logan's eyes are closed. The three kids creep up together from around the corner. Roman jumps out aiming the water gun at his father. "Pow! Pow!"
It takes him a moment to realise that something is wrong. His siblings have disappeared. He can hear them giggling from their hiding place. He's the only one who sprayed their father, who is slowly getting to his feet.
"What are you doing, Romulus?" he asks, dangerously calm.
"Uh, it was Kendall's idea." Roman looks over his shoulder. "We were all gonna do it together. Sorry."
He's bracing himself for a beating, but Logan merely sighs. "You're a moron, Romulus."
For some reason this hurts more than any slap could. He stares at his father, looking for some sign that he doesn't mean it, but Logan just shakes his head. "Go and find the others. Stay out of my sight for a bit."
Roman is deflated as he walks back to where Kendall and Shiv are still snickering around the corner.
"Thanks for that, guys," he says.
"Oh come on, it was funny," says Shiv. "It's not like he got angry."
"Yeah, he was fine," says Kendall. "Nothing happened."
"Yeah," says Roman, still feeling like he might cry for some reason. If his father had hit him he could expect a show of affection later. Instead he had dismissed him completely. "Nothing happened."