keeping lid on annoyance at all the stereotypes If your DC is inspired to learn Mandarin then there are, as with everything, problems and opportunities.
Problems - It is true that the standard of Mandarin teaching in some schools leaves something to be desired, the demand has outstripped the supply of good teachers. I know some schools do not teach Mandarin because they cannot find teachers of sufficient calibre, so beware of schools adding it to the curriculum because it is trendy without ensuring they can deliver a good standard of teaching. Look at GCSE results, www.slideshare.net/petegoodman/gcse-mandarin-discussion-day-sevenoaks-school-may-11 is a good overview of what is required at GCSE. For a motivated pupil with the right learning skills learning the 150 characters to get an A* should not be hugely demanding.
It is a different language to learn, but only especially difficult if you don't have a good ear / memory. It has four tones and written characters that you have little choice but to rote learn (but then I have neither and I have managed) www.bbc.co.uk/languages/chinese/real_chinese/ will give your DC a taste to see whether they feel they have the ear and memory for the challenge to be enjoyable.
Opportunities - You don't just learn a language, the characters provide an insight into the culture in a way the European alphabet does not and that can give you an advantage not just in business but if your DC wants to go on to study that culture. Chinese Studies degrees (Oxbridge, SOAS, Nottingham especially) are extremely popular and have a very high employment rate.
Aiyah! to all the rubbish above about doing business in China. Firstly whilst there are many dialects, business with any sort of national / international context is unlikely to be done in any language but Mandarin. It is the official language of China, the characters are in any case in common and Cantonese speakers can understand Mandarin speakers, even the old grannies in local markets, though they may they may spit at your shadow as you walk away for speaking like a mainlander ! Secondly Mandarin and a knowledge of Chinese culture will make doing business in China and with Chinese people not just more effective but more fun too. The Chinese understand that being bilingual both lingusitically and culturally is important to effective business THAT is why they send their children west for their education (that and the fact that they recognise the weaknesses of the Chinese rote learning approach to education). The woeful ignorance of Chinese culture, let alone language, and the clinging to tired old stereotypes is what holds UKPLC back when doing business with China, even our PM has embarassingly manifested that. It has made us into a bit of a joke.
As to not having the incentive of using the language on family holidays, every High Street has restaurant that it can be used in! and it will make your DC into one of their most valued customers, and get you brilliant service and food!!! Amongst the twenty somethings I know most have travelled to China and many have worked there too. It is already somewhere that offers opportunity where Britain is offering them none.