The traditional 兒 pictured an infant with an open fontanelle — the soft spot on a baby's skull drawn as an open square on top. Simplified 儿 keeps just the toddler's two legs, still standing.
儿 is child (儿子, son; 女儿, daughter — with 女 and 子 you learned) and also the most Beijing sound in Mandarin: the -r ending. 哪儿 (where), 一点儿 (a little), 玩儿 (to play) — that pirate-like 'arrr' is 儿化, the northern accent's signature. Beijingers sprinkle it everywhere; southerners often skip it. Two strokes, and it decides which city your Mandarin sounds like.
Two little legs still toddling — a CHILD.
The Beijing 儿-ending is so iconic that imitating a Beijinger means adding 'r' to everything: 明儿 (tomorrow-r), 事儿 (thing-r).