Most teams we have played run a zone defense, almost all really. My team has done man to man because I feel like that is best for their long term development even though the zone defenses tend to be generally more effective at this level. Though the fact that we zigged when most zagged actually has end up helping us on defense as teams we play are more accustomed to playing against a zone defense.
In our league they required this age group to play man to man. The problem is they didn't enforce it so there was one coach that went zone all the time. I tried talking to the officials a few times about this and the other coach tried to play it off as he was playing "man" but it was more of matchup zone (which is what he called it once when discussing it with the officials and then quickly retracted). I didn't really care too much as I thought that requirement was dumb but it was the principle of the thing. If the rules stated no zone then a team shouldn't be able to run a zone.
At that age the biggest obstacle to an offense is pressure. Typically there is only 1 or 1-1/2 players that can actually handle some pressure per team. So man to man with pressure usually disrupts any offense unless the PG is good enough (or the defender is lacking) to break down the defender and get by him. Then "help" defense is usually not the greatest and you get easy buckets. I tried to stress defense in all my practices as that is the key to the game IMO. We were able to be fairly successful with that approach over the years. Unfortunately my son decided to focus on baseball once he got to HS instead of playing both. Can't blame him too much because he got zapped of a lot of his quickness in 8th/9th grade as he was having significant ankle/knee pains from growing. Once the spurt happened he was fine but basically having two years off due to not being able to move easily put him behind the curve.
I told our parents to start the season. I am playing man to man defense not because I think it is best to win now but because I believe it is best for long term development of the kids. Same with offense, that we would start with a very basic and simple offense which was literally running a screen, a give and go or a pick and roll. Again, for me it was about long term development.
For the other teams, the default was a zone.... it was just a question of what zone that they ran. One of our sister teams ran a 1-2-2. A 3-2 was the most popular. The last two teams we played ran a 1-3-1. I think out of the two leagues, a couple of exhibition games and the tournament that we came across only one team running a man to man defense.
The decision to go man to man and focus on basics on offense, though made with a now be damned my focus is on the future attitude, ended up giving us an advantage in our games a lot of times. Teams were not use to playing against man to man as all the other teams played zone. I spent a lot of time coaching help defense. At times, the boys did awesome with it and at other times they basically watched the kid on the other time go to the hoop like "not my job!"
Even in the Championship game, the points that were no from free throws mostly came from poor help defense. When we played decent help defense they could not do anything against us. It got better as the season went on but it was still not 'perfect' even to the end.
If the other teams played more man to man, we would have feasted even more. My PG could consistently beat most defenders on the dribble but with the zones he would run right into the other defenders and I had to coach him more on using that against them to pass out of it and create the opening for a team mate as the zone shifted over to him. He improved over the season on that a lot.
I should have done this at the start of the season but about half way, I drilled into my boys "Defense, Rebounding and Passing" that is how we win. As 4th graders will do... they would get lazy on their defensive posture here and there and would have to remind them but it was really awesome seeing them after a season of my drilling it that when they were locked in, they were down and dirty with knees bent, backs up, gliding side to side with their hips facing their man as I went over and over and over with them in the Championship game. I didn't have to say anything all game long about it other than "you guys are playing great defense, just keep aware and help defense if we get beat"
My son has never done baseball. It was never a conscious decision, just never worked out. Soccer and Swim during spring/summer make it hard. Saturday, as he is playing a baseball game on his mothers phone, says to me "Daddy, I want to try playing baseball." Oh good grief. I don't know if I can add that in. I really don't want him to give up swim, which I know he would agree to try baseball because just like you said... if he takes a season off, he will lose the advantage he has over most of the kids his age.
I asked the boys on the last practice what level of basketball they hoped to play. Next year. All hands go up. Jr High? All nine hands go up. HS? All but one went up which was one of our better players... all the other boys were shocked and his closest friend, our PG basically yelled at him and everyone asked why and he said he wanted to play baseball. I thought the other boys were going take him outside and stone him to death.