Oh man, my 3 1/2 year old did the SAME thing around 18 months on a Britax Roundabout. He is surprisingly strong, so pushing the red button to release the actual buckle was not that hard for him. So, I concur that it CAN be done...repeatedly!
I really wish that Britax would sell something that prevents the kids from sliding the chest clip up and down, though, because it seems like if that were locked, it would be harder to maneuver to unbuckle the buckle.
You really just have to drill into your child's head that the buckle is only to be done and undone by a grownup. I go so far as to tell my child that he is not allowed to TOUCH the red button on the buckle, and if he does, he gets a punishment during the car ride. For him, that is NOT getting to listen to his favorite songs or taking away one of his backseat toys for the duration of the ride. I periodically glance back there in my rearview and when I see him playing with it I tell him "No touching the buckle". You have to be proactive, but it will work eventually. My 18 month old hears me scolding my 3 year old so often that he has not even dared touch the buckles.

He is a good passenger.
And, from now on, make it a habit to glance back there whenever you can, just to make sure she's strapped in. There is nothing like going 75 on the freeway and feeling some little hands raising your headrest.

I've had to pull over on the shoulder of the freeway a few times now, and to be honest with you, when he does that on the freeway, he gets a spanking, and I am not ashamed to admit that.