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.
Regular stops are a requirement.
Bring plenty of food and water. Bring energy bars. My son thought they were candy - but they really gave him the boost he needed on long climbs. Water is the best drink you can bring. Soda - especially caffinated pop will give a short rush but will drag you down later.