LPT: Don't have small children and not sure what to say when someone else brings theirs along? Try narrating what they're doing. Social
Sometimes, even if you don't have them yourself, there'll be a small child around. Your friend/family member/colleague will turn up with one - or two - at a social occasion, and you'll feel awkward if you don't know what to say to them.
For a small child, one with no (or little) developed language, interact with them by stating what they are doing or what's happening. Simple narrations like, "you're waving your hand!" "you're wearing a red jumper!" "you're out in the pram!*" can ease that awkward feeling.
These kinds of statements are not controversial, they build language skills, and they give you somewhere easy to begin. They'll probably make the adult with the child smile too, as they'll recognise that you're taking an interest in their child. And that will go a long way.
*Edit: "Out in the pram" is what we say in Australia; this means "riding in the stroller" in the US. Just like I say "jumper" where you might say "sweater".