For my 3 yr old ready to break I have a simple happy mouth bit. I have usually started with a straight bar and then moved to a jointed bit if necessary.
For baby babies - the little nylon bits you can pick up are ideal, as they are thin little bits, and so not to chunky for their little mouths.
For breaking I have used full cheek straight bar happy mouth, quickly moving onto a full cheek french link. Further down the line I will go to a kk ultra loose ring.
I would go with the happy mouth bit then TBH - I wouldn't change things too much unless it is necessary.
However, as the other guys said - full cheeks is a good idea for PAS (power assisted steering LOL!!!)
I'd have like to put my baby in a straight bar full cheek happy mouth but I couldn't find one so she's got a stainlss steel full cheek french link instead
My shetland started in a straight bar loose ring happy mouth, and is still in a 2 years later. I would always start with the simplest bit, and all being well, they can stay in this. Another option for steering in the early days is to chuck a set of reins on the headcollar as well
I'm going to be starting by boy with a KK ultra as he flips straight bars (no idea how) and the KK ultra is really thin (14mm) which is the main thing for small mouths and then when his mouth gets bigger I'll move him onto the 16mm so that it isn't quite as harsh.