Under the age of five kids are proven to be safer in a harness. The loads on their necks before that is just too great to be allowed to move so much in a collision. After five, there is no data. It seems to make sense that putting the crash forces over five points, rather than three, would help anyone. You wouldn't move forward as much. And again, with a five year old, many of them do not sit properly enough to be safe 100% of the time in their boosters.
I would put the back on, at least, for the five year old, and if the eight year old fit with the back I'd do that too. The eight year old is likely not UNsafe in a backless booster at that age and size, but a high back would help a bit as well. And locking the retractor is fine. If you want a little extra assurance without a lot of extra money that works really well. If you need to lock the retractor to keep a kid in place, then you need a harnessed seat.
If you go harnessed, the five year old would have a full range of options open for weight and size. The eight year old, though, even being only 56 pounds, may be too tall in the torso for many harnessed seats. A harness is outgrown when the shoulder goes over the top slot. The Regent is the longest lasting harness out there, so put the eight year old in that (you can find it at BRU) and if there's room then ok, if not then you're out of harnesses. The five year old would have a ton of room in it, unless there's a remarkably long torso involved. Plus there would be other options for the five year old from $150-$450, depending on seat.
HTH
Wendy