I love my Ergo too! It doesn't cause back strain. That said, I haven't worn my 20 month old daughter in it since she was a little over a year...not because I can't, but because I haven't needed to. She is so independent and prefers to walk. I haven't had to wear a daycare child recently, but I think it would only be for the first month or so, so usually around a year of age or less. I would definitely not be wearing a big 18 month old dck as they don't usually need to be held so much at that age. However, I have at times used the Ergo for a child that age as a mode of transporting when I didn't have enough seats in the stroller.