> My kids are 7 (boy) and 10 (girl).
...
> I am wondering if his 7 to 10 mile days are too lofty for children this age

In my experience with my kids, yeah, that's about twice what you can expect for the seven-year-old without some fearsome whining and a really tired little fella. For the ten-year-old, maybe an occasional ten mile day, but not five days of it.