You can wait for kids to grow into the skill at 10 or 12...or you can give them a shorter racquet, lighter ball, and shorter distance to hit over the net and teach them proper technique sooner. And I don't mean teach them to softball it in...I mean teach them to smack it. As they grow, they can use a longer, heavier racquet, a heavier, harder ball, and a longer court. The skills will translate.
OK my turn,
Just a quick history on my background. Even though I have been a “professional” tennis instructor for a few years now, I do know a thing or two about tennis.
I have been playing tennis since 1978 when I was 10 (for over 30 years). I was #1 in Jr's in my country but our main concern then was patching up our courts after they were constantly being bombed, we were playing with 3/4 grip 13oz wooden frames with white balls and thats what was available. I developed my skills at New Port Beach country club during my teens, I went on to NB academy for three summers during the mid 80's and was #1 on my HS tennis team for three years. I then went on to play few years in a D1 college. I played in open tournaments in my section during the mid 90’s. I have been into customizing frames/strings ever since stepping into Bosworth shop next door during the mid 80's. I have been teaching my niece since she was 11, she was captain of her tennis team and played #1 for 4 years in HS, she went on to play college tennis but not on a scholarship (she could have but excelled at academics/arts/music as well). I have been teaching my son since he was 2 (five years now), he is 7 years old and plays USTA U10 for four months.
I do not consider myself an expert on tennis development but I know a thing or two about tennis racquet development. My niece picked up my frames which were player frames and started tennis right away on a full court. She was able to handle the full size adult frame as a very mature and developed 11 year old. OTOH my son started at 2 with a babolat ball fighter that was under 19inch with regular balls only they were very soft/flat with little fuzz on them (ball machine left overs). He went on to play 21 inch frames then 23 then 25 then 26 on mini tennis courts. Our main concern was
the proper grip size and the frames being flexible so he will not hurt his arms using the same regular soft balls. As he grew and developed by age 5-6 he was able to handle a full size frame but I wanted him to stay with a 26 inch frame. His shots were not as powerful as with a full size but his serve motion was sound and he could hit a decent serve with a 26 as well as volley better. He was not hitting powerful shots with a 26 from the back court. . By the time he turned 7, we were now playing full court with normal inflated balls I knew I was keeping him back by having him play with a 26inch. We bought a full size adult Babolate pure drive lite with a zero grip and his shots were very explosive and powerful. He was swinging out with full speed on both ends and can rally from all sides of the court. He could now handle volleying and serve with ease with a 27inch.
I could not see my son going back to 26 or lower since he is not allowed to play with a 27 on a quick start format. His swing/motion will change and get in the way if he goes back to 26 or below. If he was in GA he would not be able to play in the U12 since he will get crushed and it will be devastating if he only had losses. He can not play in the U10 QS since it will be of no benefit and might be too boring. What do we do in this case!?!?!?! See the point. He will not be exposed to tournament play at least for another few years until he grows and can handle U12, which will/might set him back.
My second son is following the foot steps of his older brother and wants to be better than him since he is always trying to catch up. He is four and has been playing for two years (two times a week in clinics) on mini tennis. We are going with the QS format with him since he is not as advanced as his older brother. OTOH, he is an introvert thinker (thinking four to five steps ahead) who is very coachable and can absorb instruction very well at a very young age (nobody can beat him at connect 4, he plays chess and is not allowed on the chess club until 7 even though he can beat kids twice his age) We did not teach him this skill, god gave it to him. So you see it is up to the individual and every kid is different with different focus and attention span. If the older brother were to do QS he will check out and lose interest very quick.
Time will tell who is going to be the better player by age 16-18. Who knows they might stop tennis all together. I got my wife to play tennis for 10 years, now she competes in USTA, she is also involved in our kids tennis development. This way if and when the kids hate tennis it will not only be my fault

. Right now our kids play soccer (spring/fall), swimming (winter/summer) as well as roller blading in the summer and ice skating in the winter as a cross sport.
Any information and tips are welcome.
TCF as well, anything I am doing wrong and what to lookout for is much appreciated.
Thanks