I am considering learning and haveing my young children learn Mandarin (along w Spanish). Two questions: Is it difficult to learn? And is it important to learn? Or will all these Chinese speak English anyway?
For your children, they could have it easier, for both languages. If they are under ten years old, the chances of them becoming fluent are higher than if they were adults.
Mandarin does seem to be a very organized language--one advantage. However, it is also tonal, something not very common in English or other Western languages.
Those who are trying to take up English deserve sympathy. English is an amalgamation of primarily Germanic, Latin, and French languages, with sundry others tossed in. Even the grammar is supposed to be a mix. And it isn't very phonetic, and verb conjugation is convoluted. Since it is normal for native Anglophones, Anglophones might consider it easy. According to non-native Anglophones, it isn't.
Even if pronouncing an aveolar trill is extremely difficult (Spanish r trill). Personally could use the uvular trill instead for Spanish r. Apparently it has to do with where in the mouth the r sound in made. In English, there are two ways, one around the aveolar ridge (the plateau between the top teeth and the palate) and the other near the bottom of the mouth. If you use the former, you could have more success with the aveolar trill. Guessing this is also why Japanese attempting to pronounce l make an r sound (l is also around the aveolar ridge).
Sorry for rambling.
If you're thinking in practical terms, it is probably better to have your kids learn the language of a country that has a significant population *and* has a comparable salary to that of the US. From that standpoint, it's pointless to learn either Spanish or Chinese, since they can hire lower-paid locals with English skills. Think French, German or Italian.