I don't see why you wouldn't be able to teach children who will probably be beginners or level A1. At level B1 you might probably start experiencing some difficulty teaching distinctions between tenses and vocabulary and at level B2 this will certainly be harder. Teaching a language requires different skills to learning it, so it'll be a question of learning the "tricks of the trade" as well as improving your own language skills. Some initial teacher training would be useful and if you could try to get level C1 yourself, even better. But if you're planning to teach primary only, I don't think you should have too many problems.

For methodology in teaching English to primary learners, try the Try, Think and Train sections on this website, You'll find lots of useful advice there, as well as online teacher training courses you could do. There is a course for Primary teachers you might be interested in!

Hope this helps