I saw in the Pischei Teshuva (184) a discussion on how to make benching easier for children. He brings the MGA in the beginning of 192 in the name of the Bach that one can say the first bracha and then another paragraph that basically shortens the rest of benching. Until what age would a child be able to use this shorter version? Also, would this be preferable to what RSZA says in Halichos Shlomo tefilla 1,8 to just say the first bracha and slowly increase as they get older?
Our minhag is to say the first bracha, and then slowly to add to it until the child can say the whole bentching, like R’ S.Z. says in the source that you quoted. This is basically the way the schools are doing it, and it depends on the development of each child.