How old are the students that you are teaching English to?
How old are the students that you have been teaching English to?
「何歳の子どもに英語教えてるの?」は、
How old are the students that you are teaching English to?
How old are the students that you have been teaching English to?
が正確に伝わりやすいと思います。
会話だと、最後の to は、実際は言わなかったり、ほぼ聞こえなかったりで、それほど厳密に言わなくても通じると思います。