I'm not sure it's possible to do a realistic kid!fic with these two and have it be entirely schmoop. That's just not what they had. Even when they showed their childhood in the show, anything fluffy was underscored with the crap they had to deal with - John being missing, Dean having to steal Christmas presents for Sam. The closest they ever got was Bobby's flashback, where he took Dean to the park to play catch. At least he seemed to get what the boys were going through.