I would:
Leave terms pretty much as they are, but capped at 7 weeks. Any longer's painful for students & staff.
Ensure school facilities are used for summer camps etc throughout the summer break. There should be a wide choice of programmes - academic 'enrichment' programmes to stretch the very able, revision/reinforcement for those needing extra help, & sports/fun/useful stuff for those whose parents just want them kept out of mischief whilst they work.
This could include lots of educational trips out for the day, & camping/residentials as well as activities on site.
In the non-academic provision I'd include cooking & everyday life skills eg. operating washing machine, & childcare - pay young parents to come in & help teach this.
Parents should be able to sign up their dc for whichever activities they think would benefit them, with guidance from form tutor, or just 'camp' stuff as childcare.
Fees should reflect use of school premises & paying the staff. However, the scheme should be heavily subsidised for those (Pupil Premium, low-waged parents) who cannot pay the going rate.
The whole thing should be staffed by any teachers who want the 'overtime' (probably younger/childless staff with no commitments, who are skint & looking to tick a few Performance Management boxes ); TAs (& should be a recognised 'point' towards subsequent qualified teacher status if that's what the TA is aiming towards); & post-grad Uni students considering a career in teaching - again, should be recognised as valuable experience to access a PGCE.
Outside providers could also get involved if the demand's there.
Once all this is up & running, it could be extended to the Easter hols/half terms too.
In fact - we already DO quite a lot of the above! But I would like to see it extended such that no child is unable to access meaningful & safe activities over the summer, whilst equally, no child is deprived of a decent 6 week break from formal schooling.