I can counter what Maggie just suggested by telling you, as a former lifeguard and manager at a city pool, way too many parents drop their kids off at the pool for summer daycare.
I couldn't even try to count how many times we had to call home due to kids being severely sunburned or without any money to buy a snack on a hot day or got a minor injury from a fall and couldn't swim the rest of the day etc, etc only to find that nobody's home to help the kid out. Pools don't have staffs there to babysit kids while they're out of the water.
But, my suggestion is to tell your girls to suck it up and enjoy the boredom. I miss being bored...
My dad worked a day job and my mom was a night nurse so she slept during the day while I was growing up...which meant we either had to keep quiet all day or leave the house.
Morning usually meant chores around the house (my mom wrote them down every morning before she went to bed and they were always different little odd jobs for each of us kids). Lunch, then I was outside until supper.
Try to see if they have any friends that are also home alone and set up a rotating "play date" at each others' homes during the week. Drop them off at friend's houses for sleepovers with plans to stay over there all the next day, then pick them up after work and bring them all back to your place to sleep there the next night.
Libraries have books, movies, music, workshops, reading groups. I wish I would have spent more time at the library growing up.