Steam trains in Tenterden - a nice town for browsing antiques shops and drinking coffee plus a big playpark for little ones:http://www.kesr.org.uk/
Or miniature railway here by the sea near New Romney:http://www.rhdr.org.uk/
Leeds Castle at Junction 8 of M20
Not sure about camp sites ... never been myself but have heard good things about this one and I know where it is, very rural:http://www.norwoodfarmnewchurch.co.uk/
This campsite is near to all of the above as well as some nice beaches at St Marys Bay ... and 30 mins from Camber Sands. Plus in this part of the world you have the zoo at Port Lympe for a trip plus Rye is well worth a visit.
Mum to two boys, born June 2009 ... and another, born January 2013