Parker, they may be legally entitled to go full-time from September, but they are not allowed to by the school. Maybe there is just too much pressure on school places?
Also, the school holidays here are often more like 8 weeks in the summer than 6 - they sometimes get the last week of June, along with the whole of July and the first half of August in the summer.
Some people say that schools are not supposed to be free childcare - they're not. We pay for them with our taxes.
Also, the time children spend in school is, ideally, time when we know they are in a social situation in a structured environment, being well-looked after and being given imaginative and interesting things to do. Some children's home lives are pretty impoverished in many ways, and they could do with spending longer at school, just to even out their life chances and bring them up to the level of their peer group.
I know teachers will hate me for saying this - because they have children too, and want to spend time with them - but I think the school day should be longer and the holidays shorter. Going a bit off track now, but I think local 6th formers or college students could be paid to come in for the last hour of the day to supervise 'private study' time, overseen by teachers on a duty rota (so that each one only has to do one day a week, say). That way, everyone gets their homework done before they go home. Imagine not having to nag your child to do their homework!