The HMRC website is pretty comprehensive about this sort of thing.
www.hmrc.gov.uk/customs/post/buying.htm
Additionally, you also need to check re health and safety particularly for child-related goods. Contact your local trading standards office for help.
Remember also that almost certainly you'll be paying the shipping, packing and insurance costs from the USA - depending on the volume and weight of product, that can be significant in cost.
So in summary if you are buying for $17, net price, you might have to pay 5% import duty (estimate), 7% shipping etc (estimate) , so it might cost you $20 - about £13. If the sales price is £30 including VAT ( £25 net) you'll be on a margin of about 90% - at current exchange rates. If the £ falls against the $ - and inevitably it will at some point - your margin will go down as you'll need more £ to pay for the goods.
If the business looks positive, you might want to think about getting the Americans to sign an exclusive agreement (according to the laws of England) with you for Europe and/or UK as, without an agreement, they can sell to anyone and everyone and those others could undercut you.
Finally, if the USA company is fairly small, they'll have about as much experience and knowledge of selling to Europe as a garden gnome has of algebra, so you'll be holding their hand a lot on that :-)
Good luck.