Wonder where you ended up, Lisa?
the whole point of Kew (and Richmond Park) is it's outdoorsiness without things like 'attractions'. If walking round in the more wild spaces of Richmond, spotting deer and getting lost in the bracken doesn't suit, then kew offers #Creepers and Climbers' house fr small children to play in, the Evolution House which is a sort of walk through museum of evolution of plants, starting with bubbling mud, good catus house, which hs some aquarium based displays underneath, the badger sett - a human-scle mock of badger sett to crawl through- but basically, it's about walking about and exploring all the outdoor spaces. Oh, the new henry moore sculptures all over the place to look at.
At this time of the year, I'd send DH to H'row on the tube (or drop him by the nearest picadilly Line station) rather than drag the kids about in cold weather!