My DD (when, maybe, 7 or 8) used to go to a holiday club that took them up to age 14 (could choose multi sport, football, arts and crafts or drama). There weren't many older children there, but a few so maybe worth checking the clubs in your area. The one DD went to only ran from 10 - 4 though, so you would need to check timings if your DTs aren't/won't be capable of getting there and back by themselves.
DD also did a week at tennis club, which took them up to age 16. There were a few older ones there, but they were all very keen tennis players and wanted to be there to practise.
Could you ask a friend to have them for a day, then you look after the friend's DC on your day off? I think it is easier to have older ones round for the day, as they don't need as much supervision.
Would work let you, or DH, do reduced hours during the holidays, so that if they were left home alone, it wouldn't be for as long?
Their school premises (secondary) might be used as the venue for clubs during the holidays. My DD did a couple of weeks of netball at her school, and I know they offered hockey too. Again, would need to get there and back by themselves.
Churches round where I live seem to offer days for teens to go there and hang out. Never used it, but might be worth having a look if your family are church goers.