My guess was the same as Phil's. 'Do' can have almost any meaning, depending on the context. We use 'do' for any routine job that needs to be done. For example, in a domestic context, 'do the beds' can mean make the beds or launder the bedding, 'do the vegetables' can mean wash, peel and cut them ready for a meal, 'do the lawn' might mean cut the grass and trim the edges. So 'do the girls' could well refer to the daily routine of getting small daughters ready for school. Or it might mean something else entirely. Maybe they have hens in the yard and 'do the girls' means 'feed the hens'. Who knows?

The word "do" could mean just about anything, so it's impossible to know for sure From the little context provided, I'd guess they have daughters, and the father is going to help them get ready for school. Just a wild guess.