If you want to ask a question in casual speech, e.g., to a friend or your child, you can end the question with no. The other option, no ka, is inappropriate. Even though it is used in indirect questions, avoid it in direct questions.
SLTD pointed out that there is no such word as *haiyai. Please tell us what you want to say in English.
Yes, that's just my guesswork.
"Am I too young to become an English teacher?", we say:
えいごのせんせいになるにははやいのか？ You can use はやい (early) in place of わかい, in this case.
えいごをおしえるにははやいのか？ is possible, too. There is a very young children, and you think it's too early for him or her to learn English.
Literally, When it comes to teach him English, (is he) early/young?
(Who is going to teach? It's you. But who is hayai? Not you, but the children is. There's inconsistency in the subjects in the sentence.)