Berlin with a 5yr old in July - recommendations?

[yummyyorkshiremummy]

Thinking of Berlin with my sister and my 5 yr old son in July (before my family beach hol with dh) as looks like lots to do. Good or bad idea do you think? any recommendations for activities, accommodation?

Good idea. As for activities, with children you can (or at least used to be able) to go up into the dome of the Reichstag (the parliament building) without waiting in the queue. With children you could go in through a side door (on the right of the main steps up to the main entrance). The dome is glass and you walk up a spiral walkway round the dome and have spectacular views of central Berlin. Our kids loved it - you can also spot lots of trains, boats, etc. The city looks like a model train set beneath - which went down well with dcs.
The zoo is good - either the western one at Zoologischer Garten or the bigger one (but not as central) in the East (Tierpark).
In July you could go to one of the lakes and play in the water/ at the beach. Strandbad Wannsee is a big, old-fashioned one, but there are plenty of smaller beaches eg. on Wannsee along from Heckeshorn. Both of these are doable on the bus but you have to research it a bit.
From Wannsee S-Bahn station (easily reachable by S-Bahn) you can also go on a boat trip with your normal public transport ticket across to Kladow. It´s nice crossing the Wannsee and there are good playgrounds in Kladow on the other side.
The Pfaueninsel (peacock island) is also reachable by bus and nice to wander round with a royal folly to see something of the prussian kings. Similarly the Potsdam castles are all quite interesting - and our children found them impressive. The Charlottenburg palace similarly.

Altogether Berlin has great playgrounds and just wandering around the centre and discovering little pockets of Kreuzberg, Prenzlauer Berg etc. is nice - a playground will never be far away.

Wow! Thanks for a fab reply Polygon! Really helpful.