You can take her into John Lewis and try the seat out there before you buy it to see if she can escape. If she does escape this type you need to speak to the In Car Safety Centre as they have solutions for children with special needs, and have seen every situation you could imagine.
I remember some years ago on MN somebody said that their extreme escapee child could escape this type of seat - this was an older style that used the seatbelt to secure the shield. The child would basically wedge their elbow behind the shield and then work it very slowly to inch the seatbelt through without engaging the emergency lock, until it was loose enough that they can wiggle out. I've never seen ANYBODY else say this, this was literally one person - and I have read almost all the car seat threads on MN in probably the last 10 years (yes, I'm very fun at parties) and a lot in various other places too - including various different FB groups. This type of seat was very popular from about 2008-2013, so a lot of posts about it. There is a post about a child escaping a 5 point harness roughly once every other week, with many respondents saying they have experienced the same. I've only ever seen that one comment about a child escaping an impact shield. (I would believe that more than one child has ever worked this out, but just to illustrate that it must be far far less common even though these seats were at some point in fairly common use).
In theory, the Pallas G (which is the one I linked) should not have this problem anyway, because rather than the seatbelt, the shield has a belt on it which clicks into two buckle receivers on the seat. The belt is locked, unlike a seatbelt.
However, it does look from photos as though the child might be able to reach these buckle receivers while sitting in the seat. So if she is liable to figure out how to release the shield by opening the buckle, that could be a problem. This might be worth testing in the shop - it might be that a 2 year old could not reach them, but an older child of 3/4 with longer arms and sitting higher in the seat may be more able to reach them, but hopefully by that point she will have better safety awareness, better response to discipline, or might have just forgotten about the desire to escape the car seat (which may have become a game).
I had one of the older seatbelt type (which are still sold) for my eldest who is now a teenager. He could not physically reach the seatbelt buckle when the shield was on, because the shield was too bulky. So only the elbow loosening method would work for the seatbelt fixed shield seats. All the other Cybex Pallas seats apart from the G i-size have the seatbelt fixing rather than the strap attached to the shield.