Oh you will dump it more than once. Trust me.
If you don't do so hot on the kids bikes maybe two wheels isn't for you. Bikes and quads are two different animals with two different techniques. I'm not trying to discourage you but you really need to be careful.
Bikes accelerate much faster than the quads you're used to. People tend to get spooked and freeze up with the throttle full open and forget where the brakes are. If this happens keep in your mind "let go of the throttle!". Brakes are in the same place. Use more back at first. The front brake can be tricky at first. Moving from an ATV where you would tend to grab a whole hand full of front brake, that same hand full of brake on a bike will put you in the dirt for sure.
A friend of mine did the same thing. KTM too! He rode it once and got so discouraged he sold it. He rides the wheels off his ATV but couldn't get the bike thing.
Although a 125 sounds small it can be a challenge for a newbie. It will react more aggressively than your 250 ATV. 2 stroke powerbands can be very tricky when your not used to 2 wheels. It's gonna feel like it wants to run out from under you. Start in an open, flat field and play with it till you get the feel for it.
Take things slow and maybe pick up one of those how to ride videos. Although it sounds dumb, you can learn alot and it will help a great deal. I get these videos for my kids and I find myself learning from them as well. Gary Semics has a whole series and covers every point of riding. Here's a site you can check out.
Gary Semics
If you decide it's not for you and decide to re-sell it, let me know.