I used a straight bar Happy Mouth bit when I first bitted mine as a 2 year old. I was going to use a rubber snaffle but it looked huge for her small mouth!
I think either of these would be fine.
Loose ring sweet iron with lozange, encourages a wet mouth and with more use will give off nice taste, copper is also good as it warms up quickly rather than sticking a cold bit in there mouths, otherwise you can warm it slightly.
Neue schule trainer snaffle is also a favorite as are french links as they lay better on the tonge and wont give a nut cracker action.
Best thing to do is have your horse dentist to check everything in the mouth is ok and establish what sort of pallet your horse has got, as well as making sure the teeth are all ok and that a bit will not cause any problems, they will also talk to you about the tongue, as all this can make a difference in time, this will help when choosing a bit.
At this age, your not doing any thing with the bit, its only sitting in the mouth, no pressure should be used, so really its just finding something that they will mouth and get used to being bridled up.
Straight bar rubber snaffle loose ring . They chomp on it like a baby. Don't listen to people telling you to put all sorts of gunk on it I was told to try black treacle and alsorts. Just be kind and have patience.