In reply to Bxb88:
Probably more at risk of harm from being tripped or pulled over repeatedly by the leash than falling a long way if worn routinely.
If you're genuinely comfortable carrying a baby across terrain you'd also want to rope your toddler for then that seems like an odd situation to get into but if so follow the instructions on the kid's harness for attaching a rope to them (probably a tie in). Personally I'd body belay with a half rope offcut (because I have it) or some accessory cord since it's not going to be any more than confidence rope for a steep slope or little rock step. If it was doing more than that I'd rethink!
If it was for small bits of easy scrambling requiring a confidence rope providing decent security I'd probably add a couple of 120 slings with screwgates, these can be used for quick direct belays or lightweight improvised (adult) sit-harnesses. Also I'd not be carrying a baby and I'd want a hat for the 2yo and I'd want to know what I was doing climbingwise, if you need the ropes it's no longer really walking so you need the skills to go with the ropes.
If it's just to keep them close and under control at all times, toddler reins harness, large breed extendable dog lead* and an added wrist loop so you can't drop it. Looks a bit mad but they get plenty of freedom, it's strong enough and there are no underfoot tangles.
*Check the hidden knot if a cord type
jk
Post edited at 11:06