Visas for school aged children

We are a Franco-British couple with three young children and are planning a year long sabbatical to Thailand.

My father and grandmother currently reside there in Chonburi province.
The visas for my wife and I would be ED visas but we cannot find any information on visas for our three children!

We are planning on sending them to a private dual language school for the duration,would this provide them with their visas,or would they need to do the visa runs?

As my father has lived between Thailand and France for the past 20 years and has a permanent residency,does family sponsorship exist in Thailand?If so how would it work?
Would a realistic income of 100 000 bahts per month be sufficient for a family of five(does not include schooling or healthcare,this would be paid in advance).

As this is in the planning stage any info or helpful advice would be most welcome.

Thanks in advance.:)