In reply to Ragnaar:
> It looks like these are all designed for little nippers - my kids are 10 and 11 years old respectively. Can they be in adult harnesses by now (e.g. no chest harness)? Re-phrasing the question, is there an age or size at which chest harnesses are no longer necessary?
I think you'd struggle to find full body harnesses for them that weren't adult's by now.
Depending on the kiddie, I don't think anyone's done any research that would tell you when they are necessary or not, by 5 though the risk on inverting due to shape seems minimal. Most centres use the type of higher tie in harness Gavin mentioned, the Alpine or centre specifc versions, by that age.
The full body type don't allow for teaching of belaying, so their usefulness is very limited to 3-5 year olds (2s if they fit one).
Camp do a kids chest harness that you can add to sit style harnesses, which would be a lot more useful if you were swapping between kids and still allows for teaching belaying (although that might still be too small for your kids).
The down side of the single attachment harnesses, like the DMM alpine is that is doesn't teach a complete tie in procedure as you don't include the leg loops.
A cheap and cheerful Wild Country Clog Centre harness goes incredibly small and has a proper tie in and expands to allow adults to use it as well, so if you take other learners with you it can do a huge job and teaches threadback buckles too. One of those, paired with a chest harness if necessary, would tick all the boxes.