Motivated children can easily outpace public-schooled children with 3 or 4 days a week at 3 hours a day. They can be very self-directed, but the trouble with it is they go down rabbit holes and they'll excel at one or two subjects to the detriment of others.
My daughter would want to spend 6-10 hours a day on art, but wouldn't want to do her French or math homework. My son just told me that alef (in the Hebrew alphabet) was the symbol for Graham's number, but balks when I try to get him to memorize his multiplication tables.
> And since they are different ages, I worry that if I taught them all at the same time, the curriculum would average into something too challenging
If there is a subject where that is the case, I do not know what it would be. Once they're past the age of 7 or 8, most or all children are capable of what they would be at older ages. I don't think, for instance, that an 8 yr old learning to read is slower at it than one learning that for the first time at age 14.
For the record though, didn't get to teach my son to read. With his sister, I tried from age three on up... every few months I'd see if she was more agreeable to it, and she wanted nothing to do with it until sevenish. She could sound words out phonetically if I pushed from the start, but with a look on her face like I was pulling teeth with pliers. So I waited with my son until he was almost seven... and he could just read. I have no idea how. My wife claims she didn't, and his sister certainly didn't either.
I think, really, that public schools doing age segregation has more to do with them trying to perfectly homogenize the product they crank off the assembly line, than it does with making sure each student gets enough learning opportunity.