We carried my (heavy!) 9 month old all over the place in our Ergo in September. If he was on the front, he could stay in it on most rides (and we didn't have to wake him up if he was asleep). I was worried it would be too hot, but between going in and out of air conditioning and my husband and I switching off carrying him, it was really fine. Of course, we're used to carrying him in the Georgia summer! I think a frame backpack would be a pain on rides and stuff, though.