Firstly, her specific TA is to be monitoring HER primarily, and the other children as a secondary thing - in other words, as long as it isn't interfering with her primary duty. If she cannot safely monitor your DD as well as the other children (which it sounds like is the situation), then she should not be monitoring the other children.
Secondly, the 25 hours per week does NOT include her break times. DS2's school has specific people that take over each day when his regular TA has her break. He is NEVER without a TA. Ever.
I agree that you need to log all injuries, with photos if possible. Any time you are told that the injury was not witnessed, I would email the HT with that information and reiterate that DD is supposed to be monitored, and you need to know how she could have been injured without it being seen if she was being properly monitored.
If it helps, we bought DS2 a lightweight high visibility vest (made out of a thin netting so it's not warm, and it fits over his jacket on cold days) and he wears that whenever they go out on the playground. It means that if he bolts, he is easily spotted as it stands out from the sea of uniformed children on the playground. He's usually pretty close to the TA anyway, but he's a runner and is fast, so it's a bit of an extra layer of safety to be able to spot him easily. We got the vest at Wilkinsons for about 69p (we picked up a couple so we have spare).
As far as not being allowed in the woods, do they have any visual deterrent up that would remind your DD that she is not allowed there? DS2's school has laminated "STOP" signs in red in places that he is not allowed to go. Obviously they still have to remind him he is not allowed there, and he is monitored constantly anyway, but I would think it would be important for her to actually SEE something that states it's not allowed IYSWIM. I also think verbal reminders and a simple reproach if she breaks the rules is appropriate as she does need to be reminded it's off limits. Bottom line though is that if she was appropriately monitored, she wouldn't have been in the woods.