In Colorado the morning and evenings are somewhat cool, even in July. But, for what it's worth, my grandchildren don't like to play outside much in heat. I can't blame them. They will play outside in the morning and the evening, they enjoy sprinkler games (but we're on water restriction this summer so we can't do that), and they'll play at the playground (just up the street) for a little while - maybe fifteen or twenty minutes - before they start feeling like melted popsicles.
Your children seem to be in a position in which they need to learn to entertain themselves. It's always been a challenge for kids to learn do, I think. With my grandkids, I have found that the less TV time they have, the better off we all are. Even when a show is enjoyable, TV seems to act as a drug of sorts; I find that they get so locked into the TV world that they have a hard time coming out of it, and they're either grouchy, lethargic, or over-silly for an hour after that. I imagine it would be the same with electronic and computer games.
What can your children do with art supplies - paper, paints, crayons, pencils, scissors? What can they do with craft supplies? Yes, thay may take a little supervision. But if there are things they can enjoy making, and can make now for Christmas presents for their aunts and uncles, it might absorb their interest for a while. Have lots of scratch paper for drawing, writing stories, making paper airplanes (show them how - they may not know what you're talking about but you can look it up online). Got any old board games? As a matter of fact, got any Legos? They're a bear to clean up, but they can engage the brain. Let your older boy play with his siblings if he wants to - it could be a good change for him for a bit. If he wants to do something past their age interest, let him. If he hasn't read about poor ol' bored, bored Milo (THE PHANTOM TOLLBOTH), find him the book at the library, or read it to him out loud (mama/son bonding).
If they're all so bored they can't get started on anything, say to each, "Here are three possibilities for this morning. Choose one and go at it until the timer goes off. I need an hour to do my own work, and then we'll set up the sprinklers." Be with them off and on, though. They like to have you around, and this is your chance to BE around.
I've been interested this summer to see that because my older granddaughters (7, 10, 12) did not have TV as a default activity, they discovered the little-kid toys their younger cousins play with when they visit (the old Fisher-Price city toys), and are enjoying those. They're even playing with baby dolls. And why not? It's fine for a hot-weather activity and a change from an older kid's "real life."