I have a niece who is a native speaker of Chinese. Playing with her is a free language lesson, and there are advantages and disadvantages.
One advantage is that you will learn all the basic words, and you won't run the risk that your instructor will accidentally use some advanced vocabulary that will throw you off. (You also learn some words that are very important to young children like butt and fart.)
Fortunately, my niece's pronunciation is very good, so it's not like I'm accidentally learning to speak with a lisp or a childhood speech impediment.
One disadvantage is that you're learning kiddie-talk: My niece uses the word for doggie rather than dog, for example.
A bigger problem is that you might learn the words wrong. While playing with some animal dolls, my niece (four years old at the time) taught me the word 頭骨. I didn't know what it meant, but I repeated it to her satisfaction. During a break in play, I asked her aunt, "Hey, I just learned the word 頭骨. What does it mean?"
Her aunt didn't know either. She went back to my niece for clarification. "What did you just teach Uncle Raymond?"
After some discussion, her aunt figured it out. My niece got the word wrong. It's not 頭骨; it's 骨頭. (It means bone.)
Apparently this mistake of flipping the syllables of a word is not exclusive to Chinese. Her aunt asked her, "What do you want to eat?"
— Apple pie! was her reply in English.
"There's no apple pie here."
— It's right there!
"That's not apple pie. It's pineapple."
My niece patiently explained, "Grown-ups say pineapple, but kids say apple pie."
Bonus chatter: The nieces are encouraged to speak with me in Chinese rather than English. They take this as an opportunity to tease me by asking questions they think I can't answer by using words I don't know. Last night, one of them asked me, "When we get to your house, can I hit your butt?" She thought she was being so sneaky; apparently she forgot that she's the one who taught me the word butt.