I believe the common form is "X is child's play (next to|compared to) Y".
Don't worry too much, I always enjoy little gotcha's like this. It's like your brain has to think twice to understand the meaning, but in the end it does. It's really fun and colourful to read half modified phrases and that's how language evolves.
English is not my first language, and it is a rare construct. I thought it was correct.
Out of curiosity, how should it read?