Car seat in the trailer would be preferable IMO. I've seen it done and I don't see the harm in it as long as you're not going too fast or braking too suddenly. I'm guessing DD will be riding by your side? I think the 1 yr. recommendation came about when it hadn't occurred to anyone yet that they could strap their car seats in it for children younger than that who are not able to sit fully upright and whose necks cannot support the additional weight of a helmet. Were anything to go amiss, the trailer acts as a roll cage.