Some packs are not for everyone. Packs that offer more minute adjustments are better for long hikes. Some packs are only for infants under 20 lbs. Some packs can not be used for children over 30 lbs. Some packs offer a sun shade.
Some packs offer a separate backpack or pockets for baby supplies.
I bought a cheap pack first. It did not live up to my needs or expectations. When hiking with your toddler there are a few things you may want to keep in mind. Little kids can't keep up a pace. We found fun kid songs would help our son keep a constant pace.