We do use hyphens to form compound adjectives to go before a noun - it's the normal way of doing so. However, your examples are slightly odd. I'd use "as", not "like".
He was staring at her red-as-a-beetroot face.
She was sitting in the armchair, stroking her round-as-a-ball belly.
I think that both "red-as-a-beetroot face" and "round-as-a-ball belly" would work really well with young children. I know my 6-year-old grandson would love them. He wouldn't be at all confused. They wouldn't work for most three-year-olds, but that's because the concepts are too complex.