Green’s Dictionary of Slang

yukker n.

[yuck n.]

(UK juv.) a baby, a toddler; a new or junior pupil.

[UK]T. Lewis Billy Rags [ebook] A new kid stops in front of me. ‘Hey, is it you?’ he says. [...] The yukker makes to dart.
OnLine Dict. of Playground Sl. 🌐 yukker n. babies, pre-school age children.