Your stirrups need to go up at least 2 holes, this is why your lower leg is slipping back. When you raise your stirrup up you will increase the bend in your knee and this will help in keeping you in balance. You don't want to 'shove your leg forward', but rather learn to ride in balance with shorter stirrup leathers.
Doing the above will also help with your forward seat, at the moment you are to low to the saddle. Because you are riding to long you are pushing your self to far back in the saddle which again is sliding your lower leg backwards.
So shorten your leathers and practice riding in a forward seat on the flat, then over jumps. As a whole though your pony seems to be jumping well.
I respectfully disagree.
1) Legs slipped back NOT because of leather length - they are at the correct length. They slipped back because she does not have the proper muscle memory to solidify her lower leg at the girth. Her irons are placed incorrectly on her feet and she is not allowing her heels to do their job.
2) Her seat is exactly where it should be. Just hovering over her saddle, and is dead center in her tack. She is just where she should be.
3) Yes she does want to shove her legs forward. They need to be glued at the girth. She needs to wrap her lower leg *inner calf* around the girth, and open her knee's.
4) Again, her seat is perfect. Just where it should be. This rider has allowed her horse to lift her out of her tack - which is exactly what we need to do. Majority of riders tend to ride in a forward seat, which then results with riders jumping ahead and out.
This riders seat is perfect.
Her leathers are at the perfect length. No longer, no shorter - they are just where they need to be.
The only thing that needs to be corrected is her lower leg - they need to be trained *muscle memory* to stay at the girth.
And of course, correcting the placement of her iron, and heels MUST be doing their job.