We're still ironing kinks out in davening.
Now, we had it set as a routine since boys were quite little that you daven every day. They used to say Mode Ani, Torah Tzivah lanu Moshe and Shema, followed by ABCs and Alef Bais. We got them Artscrol children's siddurim. Eventually, things were added. Now 5 yo says Ma Tovu, Asher Yatzar, E-lokai Neshama, and all birchot ha Torah. We tried Ashrei, second paragraph of Shema, Aleinu, birchot ha Shahar, Adon Olam and Yigdal, but he always seems to fall back on core davening.
In shabbos groups, the boys daven whatever the group leader is saying.
Personally, having come to tefila in my teens, I took a long time getting to the "mechanical" level most people were on. Which was a good thing, since I cared and took time about whatever I was saying. So I like the idea of not pushing more and more davening on kids, but letting them take on more responsibilities as they desire. If you can't picture that, think back to the story of a boy saying Alef Bais with kavana.
The few things that 5 yo says, he understands and wants to say. 7 yo keeps alternating between showing off the davening that he knows to falling back to five tefilot that he actually WANTS to say. 7 yo was saying more and more in school, with the result of caring less and less about what he was saying.
I used to insist on sitting nicely for davening, open sidurim in laps, saying every word, thinking that you are talking to Hashem. Now I let them march/dance around, preferably with siddurim. I do a silly line before Shema : close you eyes and think about (pizza, walking on the ceiling, zoo). They yell back: no, we think about Hashem, who is one and only G-d, and we love him very much. I believe that at least sets kavana for Shema.
As with potty training: some kids get is earlier, some later but no one gets married in diapers; so with davening: by bar mitzvas, they will get it.
A sign that it's working is that 2 yo insists on davening too. She gets a sidur and does say Mode Ani, Raishit Chochma, Torah and SHema.