I have carried my 17kg 4 year old on my back in a manduca. The only thing was that he is quite tall with long legs so his legs weren't really well supported, they kinda flopped about a bit. If you are going to do it as a regular thing, I would look at getting a specially designed carrier so that they get the leg support to achieve an M position, and less of the crotch dangling effect.

Weight wise, it was fine for me to carry 17kg, but I am pretty used to hefty stuff and have a large frame myself.