If learnt young enough, children can learn a second language as well as their first, including languages as different as Chinese and English. All languages transmit meaning. Albeit meaning can vary with cultural differences, hence why the colearning of language and culture, including Chinese, is important. For children languages are just different ways of communicating meaning.

Children learn languages easily as their brains have yet to hone in on one way to communicate. For them the differences between Mandarin Chinese and English are fun as they appeal to their curious minds.

These differences make for cognitive development - learn another Romance language such as French or Spanish and the brain develops but learn Mandarin Chinese and new pathways spring up everywhere to cope with the huge difference. Once a child learns another language other than their first language, third langauge learning becomes easier.

As parents or teachers, children will always surprise you as they learn a new language easily. Making mistakes, they learn from them. Seeing words in different contexts and playing with sentence patterns is all fun and games for them. Even logographic Chinese characters are interesting.