I can see that not everyone has had such a positive experience of ESF. Maybe our children flourish there because there is not an enormous diffence in culture between French and Belgian French and our kids speak French.
ESF does have adult-child ratios, for under 6s it is usually 6 to 1, 8 to 1 during school holidays. I've only seen groups of 12 maximum even in the older levels, with 8 to 12 year olds who are competent skiers.
Many ESF schools let you reserve just the first lesson, then if there is space, you can pay for the next 4 or 5 days. This is a practical option if you are skiing outside French school holidays, ie Jan and especially Februay. That way, if your children hate it, you have not lost a whole week of fees. But my friend (Belgian) got a full refund when her son refused to ski on his first lesson, so it's definitely at the school's discression to refund or not.
Do remember that ESF is French instructors, we can't really expect everyone to learn and speak English competently. French culture is very geared towards children conforming and doing what is told, it can be more of a culture shock to the parents than the children. French people are no less caring than any other nationality, they simply express themselves and act according to the culture they are brought up in.
From experience of the ski school in les Houches where we are going to again this year, the instructors there rotate around all age groups, except for the most senior instructors in the school who can avoid the yonger children, meaning a few instructors spend the entire season with the little ones. Our 5 year old is flocon/1ere étoile this year so she'll not be with the lovely instructors she skied with last year, I found them all personally very caring towards the little ones, ratio was about 4 to 1 in mid April, lots of attention, daughter could have stayed there all year.