Concur, concur, concur.
Just to be clear, the area entirely past the stadium is for the minor leaguers. There is limited but fun opportunity to interact with a few major leaguers when they pass between the one practice field (batting practice before the game, mostly) adjoining the stadium, but if you have your heart set on one major leaguer it may not work out that day - they do have their perks at that level of achievement, and aren't fully accessible. I enjoyed watching Alex Kirilloff interact with a few kids, in transit, last March. You can also watch from behind a barrier fence into the batting cages.
I gravitate toward the minor league fields. There are warmups and drills during the mornings, and then other nearby teams send their spring squads of A and AA and AAA players for early afternoon games - usually two games running concurrently, and I've seen instances of three games at once. The fields are set up in a cloverleaf shape so that you can be in the middle and pivot from one game to the next as your interests dictate. Oh, bring water and snacks/lunch because there are no concession stands back there.
There's a simple rule of thumb: eye contact. Inside the playing field, the players rarely make eye contact with fans, so keep to yourself. If you see them looking around for interaction, you can sometimes engage them through the fence, but take it in stride if you miscalculate and get no response - it's not rudeness, it's discipline.
If your 5-year old is outgoing, help encourage him to walk right up to one of the players in the public area, at an appropriate time during a game or drills. During a game some of the ones who aren't playing sit in the metal stands behind home plate of either of the back fields. Or if you're there in the morning during drills, he can take a chance on approaching a player who is moving through the public area, from one field to another or from the fields to the clubhouse building. Either way he'll likely get at least a friendly acknowledgement. I'm not the outgoing type so I didn't bug Jose Berrios back when he was a single-A player, but then I wasn't a kid and being a fanboy at my age wasn't my style; I still immensely enjoyed the atmosphere my own way and it remains a fond memory - the personality we all saw on TV was the personality he had back then. If your kid talks to some 19-year old God Who Walks The Earth In Metal Cleats, he'll follow that playing career all through the minors.
Go ahead and buy tickets for the major league game inside the stadium (get them ahead of time, they do sell out), and he might surprise you how long he stays interested in it, if he got a taste of things on the back field the day (or morning) before. Once the starters get pinch-hit for, you might be ready to leave before he is anyway.
"Is This Heaven?™" "No, it's the fields behind Hammond Stadium in March."