get a booster with a back part. i put my son into a graco rally sport at just under 3 years, due to height issues.
put both seats in theback of your car (i am assuming you have space)
put her in the booster. tell her she has to sit still, and leave the belt alone.
if she touches the belt and buckle, she goes back in the baby seat. which is already set up.
if she does touch, stop the car and move her as soon as is safe.
if she has a toy or book, she should have no need to mess with the belt.
my son is 8 now, and still fits in the rallysport.
if we are on a long trip, he goes in the rally. if we are local, he has a regular booster.
if you are in the uk, go into a large mothercare store. they have seats out on display for you to try.
sit her in a few. the mothercare own brand looks ok. but the graco came down just a bit lower when my son was small.
i don't agree with lying to people, but i would be very tempted to say the girl is 3 if they ask. well, she nearly is!
also, if she gets to choose a seat( the rally has a couple of different colours), there might be less tantrums.