I echo Bluebell, - I was a lead officer for child protection, and in training this thorny issue always came up... there is no legal age at which a child can be left alone, or babysit, as this could be used in mitigation if something went wrong.
15 is old enough to be left alone, and possibly to look after an 8 year old, in exceptional circumstances for short periods, but all day impinges on the older child's time, and if, god forbid , anything happened, ( accidents do - mu older boy broke my younger ones cheekbone playing with a balloon...) think of the repercussions.......
Parents must ensure adequate care for their children, so in this case I doubt the care would be deemed adequate
Most areas have after school and holiday clubs, and a childminder would be happy to take an 8 year old- I used to childmind and had children as old as 11 - I used to do annualised hours so the parents werent hit witha huge holiday bill, and nowadays this also make it easier when claiming childcare