However, there's a fine line that can't be crossed because of the incendiary nature of the situation.
While doing wind sprints, he asked his father, "How many do we have, Dad?"
His father replied, "You had three. Now you've got seven."
The message was loud and clear.
"When we're at the field, I'm their coach, not their dad," Dave said.
Fast forward to last May at Dodger Stadium, where Vista Murrieta had just won the Southern Section Division II baseball championship, 1-0, in 11 innings. On the field were Derek, his older brother, DC, the team's starting third baseman, and their father-coach, all locked in a hug.
"It was pretty much indescribable," Derek said. "To have all three of us there was very special."
Credit Dave Legg for seemingly doing everything right when it comes to the father-coach balancing act.
He was a college assistant at San Diego State and then at Wyoming when he left the profession to help raise his family and coach his sons in youth ball.
He took the Vista Murrieta job five years ago, just as his sons were reaching high school age, and everything has worked out beyond his expectations.
DC has become the starting third baseman at Utah as a freshman, and Derek, who signed with Long Beach State, is one of the best senior shortstops in the Southland, hitting .460 for the 11-4 Broncos.
To say that Dave has been having fun hardly explains the pure joy of watching his sons grow up right before his eyes on and off the field.
"It's been something I could never replace," he said. "It's been the best thing I could ask for."
Derek has enjoyed every minute of sharing his athletic ups and downs with his father.
"Right after the game, he turns into being a dad and never takes the game home," he said. "It's very good to know the separation."
Derek has benefited from being around baseball all his life. It shows in his preparation and the way he approaches each game.
"I like to say I'm a student of the game," he said. "Most people just play the game and don't sit down and actually watch. In my opinion, you have to watch to get better at it. I'll watch a game every night if I can."
His father is proud of what his 17-year-old son has accomplished the last two seasons.
"From the coaches' side, he's done things in practice and on the field I haven't seen before," Dave said. "You go, 'Wow.' His knowledge of the game is far beyond some of the kids."
It's always helpful when the son of the coach has talent because that negates complaints of favoritism, and there is no doubt the Legg boys earned their way into the Broncos' starting lineup.
While these are the best of times for Dave, Vista Murrieta's final game this season will produce a moment he's not looking forward to. It will be the final time he gets to coach his son.
"I've only allowed myself a short time to think about that because I don't want to," he said. "It's going to be very hard."
But the good news is that Legg has become a standard-bearer for the father-coach, showing how one can survive and even thrive in the dual role.
As Derek put it, "He'll still be my coach, just not on the field."