CertTESOL - Will I be qualified to teach children as well as adults or will I need an additional qualification?

The teaching of young learners, especially those aged between 7 and 12 years old, requires more specialised methodology, materials and means of assessment. You can learn more about specific Young Learner pedagogy on the TYLEC (Teaching Young Learner Extension Certificate). 

If you are teaching in private schools, the CertTESOL should be sufficient to secure you a job. Should you wish to teach in state schools you would need to have a PGCE qualification or equivalent, or you could work as a teaching assistant without PGCE. 

You should check with government representatives in the country where you are planning to teach.