Teaching them to smile is pretty easy. The way I teach is tickle the inside of their top lip until they lift it (move their lip away from the pressure) then release (remove your finger). Pick a word that is going to be your release and say it in a higher happy voice, something like alright, or ok when you remove your finger say the word. It is important to have a release when they get good at the trick they will be expected to hold the trick until told to release. I try to stay away form using food to teach tricks it just causes distraction.
I am sure if you look on Youtube there are lots of ideas there. Look for how to teach yes and no, give a hug with their neck. I would stay away from tricks that involve the horses feet leaving the ground like counting, picking up the foot to hug, do not teach a horse to rear, I don't like teaching them to kiss their teeth to close to any body part not a good idea ect.
Smiling, yes/ no and give a hug seem pretty cool for a young girl to be able to do safely.