You know... even though conceptually I'd be prone (pun!) to assuming all babies (even those not missing legs) are Crawler Zombies, rules as written they'd have just an equal chance of being the standard Sloucher or Fast Attack zombie as any human...
Meaning that even though normal babies can't walk (much less sprint) the magic which reanimates them would give them this ability.
They'd also presumably have the same motor skills and intelligence... this is a step down from adult humans on average, but consider that Thinker Zombies can fire guns and Mock Zombies can even talk and reload weapons...
By the book, there's really nothing preventing a baby-turned-zombie from becoming a Fast Attack / Thinker / Mock Zombie and operating just as competently as an adult-turned-zombie would in every single capacity.
So you'd have Baby Mock Zombies who never age but are as intelligent as adults, driving cars, firing guns, etc. But of course, they could also pretend to be a normal baby long as they didn't rot...
I'm thinking they should probably have less PS though since rather than being an assigned-dice it happens to be based on % of strength while alive (doubled).
Baby zombies would have exactly the same PE / HP / SDC as adult zombies though

They would also do exactly the same amount of damage with bites and headbutts, even though they lack teeth and don't have fully formed skulls. You could even have Flesh-Eating Baby Zombies who somehow manage to eat human flesh despite a lack of teeth!
Unless of course one of the supplements specifically addressed this situation and gave them lower stats? A cosmetic effect where a Baby Zombie Moan sounds like exactly like a normal crying baby would be excellent.