V.J.
K.-
I think we have twins!! *L* My daughter started this when she was 2 as well. I switched her car seat to an overhead. (the kind that have a bar that comes down over their head and positions in front of their chest). This works because she can't access the bottom buckle at all. As she got older however (now 4-and TINY for her age, so we'll be in a car seat forever!!) she learned how to unbuckle the top and climb out that way. I finally took her to the store last month and let her pick out her own car seat (with my approval of course) but now I can tell her she picked it out, she has to stay in it and so far, it's worked out very well! She chose the "eddie bauer prospect booster car seat-highland" convertable car seat which is very reasonably priced at babies r us. It has a removable pillow, headrest and back, so when she FINALLY reaches booster size, she can take all the pieces off and VIOLA she has a big girl booster, as well as comfy covering, movable arm rests and (my favorite) built in cup holders on both sides.
Best of luck!!
V.