2 years in Paris or London will be "foreign"...but you will understand the language with no training in the UK.
Despite what you have heard...Most french folks in the "real" parts of Paris are ok with Americans who don't know the language...but in the tourist areas...Americans are not that well liked...and, quite frankly, alot of it is for good reason. You can easily survive without knowing the language.
Kids...you will want them to attend ASP or ASL (American School of Paris or London respectively). This way they don't need to adjust to a different style of education and then re-adjust when you return. HOWEVER, make sure you get tuition included in your expatriate package. These schools are not inexpensive.
If I were to choose between London and Paris...I'd probably choose Paris...there's just so much more to Paris than there is to London. And 2 years is pretty much a >visit< span.
BUT..as was pointed out before...its not far to Paris from London...so if you want to live where you can understand it...but take the train to Paris for a few weekends...you could do that too.