Mandarin moment

I was having a hard time understanding what my classmate is saying, because his accent is not great, and he doesn't do tones. (Good thing: I have more classmates!)

The teacher said "I sort of understand what you're saying, but I really hope you didn't mean to say it."

My classmate was trying to say, "I bought a pair of shoes (xiezi) but they are too small."

What he actually said was, "I bought a pair of children (haizi) but they are too small."

I have THOUGHTS about language acquisition but it turns out I'm too tired to write much that's coherent.