Kid and squirt have similar vibes.

Looking at it the words (lad and lass) are likely from old norse. I had a look at some old norse words starting with "la" and didn't find much that sounded good. Here are the best one to two syllable ones in terms of meaning and sound I could find:

• Lauss means loose, free, or unimpeded
• Laug means bath or hot spring
• Lauf means leaf
• Laun means reward or secrecy
• Lax means salmon
• Længæ means desire (æ is close to how you pronounce the a in lad or lass in english)
• Læknir means doctor
• Læsa means lock or to close

I like laun -> lan/lann. Sounds nice, even if not meaningful. A lann is running down the street.

Edit: Also you can use it like laddie and lassie: lannie