I think you're right to keep refusing.
I'm a former Reception teacher and would never, ever suggest a child missing those first couple of weeks.
As Misdee says, suggest that you all go at the beginning of July.
Has your dad spoken to the school about taking his children out of school? The last school where I worked was very supportive of parents taking children out of school for family holidays, but would never give authorisation in September or May (SATS)