I'll try to be as unbiased as possible
A dad asks to take children away with partner and family in October half term for big family holiday. He's taking them to X.
Mother agrees and books holiday with her partner for the same week in Y.
Mother doesn't inform father that they are going to Y.
Father then changes holiday to Y without knowing that mother and her partner will be there too the same week. They decided against X as not workable.
Y is expensive holiday abroad, all deposits are paid. Fathers party is 6 adults and children. Mothers party is just 2 adults one child.
On discovery that both will be at same holiday at the same time (although different flights and hotels) father asks if mother could change her holiday as awkward for them all to be there at the same time. Would be strange for children.
If mother changes holiday she will be changed fees roughly £200. Partner of mother doesn't want to change destinations as looking forward to going to Y.
WWYD?