I agree with Liz re the Runabout/Valco and the sun - however it is easy to go from one seat to two - we bught the toddler seat when DD came and DS loved it.

My DD will be 3.5 when this bub comes, but she is teensy (like 12kg!!) and won't mind sitting up there if need be. My DS was about 17 kgs at 2 when DD was born so weight wise it was still ok but you could notice the weight shift.