Help make my deck safe

Hi! I have two kittens. They will be a year old this summer. They have been strictly inside since we rescued them from living in our car last summer. The only downfall to this arrangement is that we have a really nice little deck off of our bedroom that we used to keep the door open to. I love it because I could let my little 3 year old daughter play outside while I'm inside and I know she's safe. Last summer we didn't use it because I didn't want the kittens escaping into the roof. Any ideas how to kitty proof it so they and my daughter can safely play out there and get some good vitamin d? It is second story and there is only about a foot or two of roof that they have access to

My cats are indoor/outdoor and the places they can get to that I thought they wouldn't be able to? Amazing. My girl goes 7 feet up the side of our aviary (which is chicken wire on the sides) to jump to the birds sunshade and climbs backwards back down it. My Moms cat will jump to the top of a 5 foot tall dresser without any assistance or ledges on the sides (and the drawers are flush). My boy will climb 10 feet straight up and jump the 2 foot gap to get on the garage roof. Add on curiosity of seeing the roof and I could see the kittens being curious and overcoming most obstacles. 

My immediate thought would be a netting or lattice that goes all the way from bottom to the top of the balcony area. Secured at all points because if you give them enough space for their whiskers they will get through. I think that Beth Stern has some sort of enclosed balcony on her Instagram that you may be able to copy. You could also try a roller or hooked set up on the top of the balcony's existing barrier but I'd be worried about them rolling the wrong way or figuring out a way over it. 

If there is any way to get around it I know my boy would find it so it would have to be pretty escape proof to hold him. Honestly, if it was me, I would probably build a catio enclosure on part of it just for simplicity and to maintain human usability and enjoyment of the area. Barring that, leash train the kittens and take them outside on leashes when desired.