Can my husbands ex-wife keep their 2 children out of school? They are 6 & 8 yrs old. Is this illegal? If so what can we do?

My husband and I are having an issue with his ex-wife who is the custodial parent of their 6 & 8 year old kids. They share joint legal & joint physical custody. She is trying to keep the children out of school for the next 2-3 months while she relocates. She plans to teach them from home, but not thru a recognized school, just as she sees fit. My husband thinks this is illegal due to the children's ages. We want to know if in fact it is illegal to keep them out of school, and if so what options we have to do something about it. Can he get temporary custody and enroll them in a public school near us? We live 2hrs away from her currently. Any advice would help!