Possibly the Greenline 701/702 which goes from Slough Bus Station to Legoland. However I don't understand how the Rainbowfares work... they seem to give a price to London but not for just Slough to Legoland.
I think this (PDF) is the pricing structure. I think as long as you take 701/702 from Slough Bus Station, then it's £6.30 Adult, £4.20 child (for Bracknell 1,2,3 FirstDay ticket). I find their pricing very confusing though, so take a look yourself and see if you can work it out.