I am not convinced, despite how nicely his solicitor's letter is written and set out (it's very conciliatory) that the changes look like they are in the best interests of the children as the changes seem to be primarily needed to accommodate the father. If that is the case, a court would not necessarily make the changes as anything the court orders must be in the children's best interests.
At 7 and 10 your children are very young to have their views be a determinative factor in court proceedings, but they are certainly old enough where their views and opinions will be sought by a CAFCASS officer if the court decides to order a s7 report, as to what it is they would like to happen.
It seems dad wants to avoid having any contact with you - which is his prerogative, but in that case, perhaps a pattern of Friday after school to Monday morning return to school on alternate weekends, with a collection from school and a return to school one day in the other week when he doesn't have them on weekends. This is a very usual pattern for children of school age as it allows the non resident parent to have a weekend with the children but also to have regular contact with the school. Perhaps you could suggest this to the solicitor as an alternative given the children's wishes and feelings which they have expressed to you. I suspect they won't agree, but you are at least engaging with the discussion process. Father will have to attend a MIAM (mediation information and assessment meeting) before he can apply to court in any case - if you agree to attend mediation, then you can discuss your preferences there. If you both can't agree at that stage, he will need to apply to the court. He's not going to get a hearing this side of Christmas though - unless your local court is not as busy as other courts I know of.
So to be clear - if this did end up in court, your children's wishes and feelings would definitely be sought by the court and would play a factor in the decision making, but would not be determinative. Any emotional or anxiety issues the children are displaying would also be investigated a bit further to see what is causing it.
Does that assist?