In reply to Pbob:
It's not quite clear (or perhaps it is and I'm still caffeine deprived) whether you have a sit harness you wish to supplement or whether you want to improvise the whole harness.
You could look at a bowline on a bight (bowline rescue chair?), it makes a reasonable improvised harness with leg loops and a waist or chest loop which is worth knowing but it's really not ideal for your kids first taste of climbing. Borrow or buy a proper harness.
If it's to supplement a sit harness then as already mentioned, the Parisienne baudrier is what you're after.
jk