I'm not sure what champions are on free rotation for low level players, so I'll just try to stick with cheap or simple champions.
[Also, make sure they know that they can get Tristana, Alistar and Garen for free from Facebook, Youtube and Twitter]
Also, make sure you stress the importance of Tier 3 runes when they get a bit more experienced, so they don't waste money on Tier 2s.

Jax (1350IP) is a pretty straightforward bruiser. At early levels, a lot of champions don't really know how to bully him in lane and he scales well with relatively simple mechanics. Plus, a lot of low level players will just sit there and let you whack at them.
Kayle(450IP) is a strong lane bully and has good utility, so they shouldn't have many issues laning with her. She can also serve some useful mechanics (kiting, chasing, positioning) with some fallback mechanics to keep him safe (a heal, a slow, a movespeed buff, and invincibility).
Annie (450IP) is a basic mage who is somewhat durable, teaches you how to count your spells and manage mana, and who has easy-to-apply CC.
Sivir (450IP) is a good beginning adc because of how easily she can clear waves, her passive makes kiting relatively simple, and her ultimate can easily allow your allies to chase or escape.
Mundo (1350IP) is a strong tank with one skillshot, a lot of sustain, and a lot of mixed damage. He can teach them how to land skillshots and the importance of managing your health. Also, the importance of occasionally sitting back in lane to heal, and your ultimate is a really good safety net.
Morgana (1350IP) is a good mid laner who has good waveclear, sustain, and safety. I believe that she would teach the importance of using minions during laning phase (trying to aim bindings around and between them, people using minions to block skillshots) and in teamfights (landing your ultimate).
Taric (1350IP) is a basic support with durability, a heal, point-and-click CC, and useful auras. Taric can teach you how to position in lane and teamfights to scare enemies or protect your allies, catching out enemies and healing allies. He's also strong enough to 1v1 a lot of champions after a few items (Face of the Mountain, Sightstone, Iceborn Gauntlet, Locket of the Iron Solari).
Looked up the free champions for low level players. Of them, I would recommend Shyvana, Wukong, Varus, and Lux for simplicity. I still believe that those I suggested are probably better though.
