This is something I've translated frequently when it's come up in descriptions of Spanish hotels that provide led on-site activities for families. I'd call the job "children's activity leader / monitor" and the school "children's activity training center" - even though two words (actividades/animación) are used in Spanish, because 'entertainment' can be misleading (it is a range of fun/sports/creative activities, and they are not learning to be clowns/children's entertainers either!) I'd also prefer 'children' to 'youth' as the former tends to be used in the UK to cover anyone up to age 16, and 'youth' has a ring of 'social programmes' about it... suerte!