Give the kid a hat and some sun glasses. Better yet, attach a small sunbrella to the car seat.
There are shades that attach with suction cups so they can be placed just where you need them. Maybe something like that would work on both the side windows/rear window. Here's a link to an Ebay search for car shades.
2004 E60 530is, Delivered 12/12/03, TiSilver/LGrey, PP, SP,PS, PDC, ACC, FRS, Step, Xenons, clear bra, body colored rear inserts. (Retired, 1983 henna red 320is)
2004 Corvette Z06 Commemorative Edition