l wonna build a wall for the kids is there some thing l can do at the top of the wall like have some kinnda pully with rope to keep them safe.

A couple pieces of information would be useful here: 1) age of kids, 2) size of wall (height by width). What is climbing experience do u and/or the kids already have.
l am going to assume that the kids r older than 6 yrs old. If u r building a wall that is only about 10ft tall then a big thick mat (crash pad) would be more appropriate then ropes. If the wall is significantly taller then u will need climbing rope, harnesses, & top rope anchors. Place the anchors at the top & run ur rope through those. Reinforcing the anchors r a good idea as well. 3/4'' plywood wont last long. It is better to have alteast a 2x4 that is attached to the framing of the wall that an adult can easily hang on w/o it moving at all. Then, go find an indoor gym somewhere to teach u how to belay. A crashpad or 2 is still a good idea so the kids can still boulder w/o needing a rope.
My suggestion would be, however, to get a membership to an indoor climbing facility near you. Not only will that get u experience climbing. It will also give u better ideas on how to build ur wall by talking with the staff there.