I've suggested Tate Modern + river boat. I think DS would enjoy both and the Tate is free. Aunt will be staying in London, but DS and I have to travel in by train and we've got a direct line to London Blackfriars, so it's also really nice and easy to get to. However, aunt doesn't seem very keen on modern art and has suggested London Zoo, but it's £40 just to get in, so bugger that! DS only went to a zoo at half-term anyway.
I used to live in London, so feel like I know it pretty well, but if anyone has any bright ideas for how to entertain such a diversely aged party without spending a fortune I'd be very grateful. Large amounts of walking are a no-no as aunt walks very slowly and DS doesn't have much stamina over long distances. I'll be taking DS to the Natural History Museum on another day as he loves dinosaurs, but aunt doesn't believe they ever existed , so that's not really an option with her!