Two things I would identify for serious consideration are:
1. the legal basis on which to practice may be quite challenging once you return to the US
I am a Canadian trained (BSW) social worker and until I took a social work law course in the UK, my knowledge was really not as adequate as I would like to practice effectively to support clients
2. because registration in the US, AFAIK, is the same as in Canada, which is that it is state / province / territory based, would you need to provide inter-country transferability every time you moved from US state to US state in the future - if so, for me, this would provide an extra barrier to wanting to do a British MA / MSc in Social Work, because of the extra hassle
On the plus side, if you want to do child protection, at the moment as it is on the job shortage list, you could get a work visa, (though this is always a changing beast that you cannot rely on), if you did decide to stay in the UK, it may be possible.
That being said, it is not usual for foreigners to take a postgraduate social work course, but it does happen (this is because there is a bursary system from the registration body, which most non-EU citizens are ineligible for):
http://www.guardian.co.uk/education/table/2012/feb/28/social-work-postgraduate-masters-courses In the meantime, I'd consider strategic volunteering for whatever you decide to strengthen whatever path you decide on. Good luck!