We went there on Saturday as it was their first birthday and it was free admission. It is all about the history of the docks and has interactive exhibits. Downstairs there is a free soft play area and some other childrens activities. The usaul admission price is £5 for adults (but the ticket lasts a year) and children go free.
The nearest tube is Canary Wharf. It makes a great day out as it is in a great docklands setting and could be combined with a trip on the Docklands Light Railway. Also the shops at Canary Wharf are good.