I recall when my boy was 9 or so when his coach felt it was time he handed me a few sets of string and said, "He is ready for this". "Please have his racquets strung at such and such a tension". From there on I ordered more string and strung his racquets per the instructions. A year or so later, he would come up and hand a different set or set(s) of string and say. "He is ready for this." "Please have his racquets strung with this in the crosses and this in the mains at such and such tensions"... I just bought more and strung per the instructions. etc.
He did the same with his racquets... "He is ready to demo new racquets, try this and that, and others in this weight class, specification". We went, he hit with various racquets, he demoed some with his coach, and then we worked on finalizing what his next racquet would be and purchased some.
I thought all coaches did this. I am not clear how it is possible to recommend a setup for a child that age without seeing how they play, their racquet, how big they are, how frequently they play, etc. With more feedback perhaps it is possible to provide some options but wouldn't it make sense to seek input from his coach and then perhaps ask here on how those strings worked out for folks here with our kids at the same age?