I think it's better if the child learns how to pray and gets used to it. I wish I had someone to teach me when I was little. If the child starts praying early he'll get attached to the Salah and be less likely to leave it. I remember when i was about 8 I started reading my grandma's Quran, didn't understand a thing but i loved it. So my grandma gave me that Quran and it has a great sentimental value for me. Whenever I can I open it and memorize and recite verses.