If the CELTA or some TESOL cert. is not a prerequiste for the job - and it appears you have the job already - your PGCE will have already armed you with the knowledge you need of teaching practice. What you need to know about teaching English you can brush up by reading either:The Practice of English Language Teaching
Jermemy Harmer 2001
(it's practically every TESOL course provider's bible)
A Course in Language Teaching
Penny Ur 1991
Cambridge University Press
Then go to the Longman, CUP, and OUP catalogues and check out the course books for kids. If you are allowed to make your own selection in the Prague school, if any books take your fancy, you are welcome toask me again for my opinion.