You might want a quiet day after two at Drayton Manor! Ours have always enjoyed steam railways, childrens farms (eg. Ash End House at Sutton Coldfield). Isn't there a Sea Life Centre in Birmingham too?
has to be thinktank here which is the science museum. They have a whole area just for littlies, its like a complete toen but at their size- a cafe, a doctors, a massive role play really!! Also have a real digger arm you can work and a water thing. Lots of it too old also but well worth a visit. Walkable from city centre