Took 4 x DSs (not all mine!) 6-9 over the Easter weekend. The Planetarium had just opened and it was fab!
Kids cost 4.55 or .95, can't remember, adults 7.95 + 2 per person for the Planetarium. Intech is a hands-on science museum which'll keep 'em occupied for 2-3 hours, pulling, pushing etc. The planetarium show we saw was Black Holes (poss the same shows at at Greenwich?) and they loved it. There's a warning about it overwhelming young children which it might (saw a nervous 4-5 yr old?) but my guys loved it. We went on Easter Saturday, arriving at opening time. By 1pm it was HEAVING but they still got a go on most things before they reached the end of their attention spans. The cafe is also fine but get in the queue earlish to avoid a big wait.