We had this problem with our youngest.
We use a buckle guard cover and a safety clip.

This might be a gory answer and some will no doubt judge, but I told my kids that if they don’t have their seat belt they may fly through the windscreen and then there will be blood.

Naughty girl! Maybe provide her with an activity to do whilst in the car. Hopefully it will take her mind off undoing her seat belt

Children tilt the neck (and head) down too much when using technology gadgets. There has been numerous warnings that it can cause neck and shoulder problems which may not be obvious for awhile. A picture book that your daughter can hold at a suitable option would probably be helpful provided she doesn’t starting throwing it which could be very dangerous if it hits you or lands at your feet (because of the pedals) or on the steering wheel. You may to use a small soft toy as a safer option

I can’t really think of any suggestions for this one.

I hope you were able to find a solution.

Does she undo it because it is too tight for her. My kids never ever attempted to undo it because it is a sign of going out and they just automatically obeyed . We always tried to make the kids as comfy as possible as they are triggered by heat , ventilation etc. Maybe do a spotcheck and insist she must wear it otherwise she is going back home .

Hmmm, not too sure other then repeatedly telling her no, stopping and checking it. Maybe if you gave her something to do to keep her occupied, she might be less inclined to play with her seatbelt

Houdini straps are great!

One of my daughters did this, you can buy a Houdini strap which stops them getting their arms out you can also buy online things to stop them from undoing their seat belts from car seat belts to a normal belt. At first when it happened I stopped on the side of the road and told her we won’t be going until she did her belt back up I explained to her that it’s dangerous and that the police wouldn’t not be happy and that mummy would get in trouble. One day I was near a police station when she did take her belt off and I went into the station and got an officer to explain how dangerous it is and that seemed to work for a period of time before getting the second buckle stop so she couldn’t get it undone however this last for two months then she was able to work out not only the hudni strap but the buckle stop as well.

As others have said, Houdini straps are great and Infasecure have a new Securall that is brilliant too. http://infasecure.com.au/products/securall/

Houdini straps

distraction techniques like apps on a touchscreen device because all kids love touchscreens- get her fav show/movie/character apps and only let her have it in the car so that she won’t get bored of the app.

car pack with snacks,games, books, pencils etc in a backpack that stays in the car. Good to build up collection of items and they get freedom to play with what’s in there.

(also there are heaps of free apps)

Yes to the special straps, and tell her the car doesnt move until seat belt is on. (and STAYS on).

Houdini straps

What the other mums have said, I know there is something you can buy to stop them, not sure of the correct name.

You can get covers for them!

Go and ask at your baby store. They have covers which go over the buckle after your child her in.

there are some good comments here

i hope u found the solution!

Houdini clip or there is a mechanism you can buy that beeps if the seat belt is undone.