There may be devices that would fit in the door, but I'd be reluctant to try to use them on the ship for fear of damaging the door. They would also make it difficult for you to use the veranda, unless you intend to use it only when your son is not around to see how you opened the door.
In addition to opening the door, someone needs to be strong enough to open it. It's possible that would be enough of a deterrent if your son is young or small.
If you are concerned that your son will be able and likely to open the door, head out to the veranda, drag a chair to the rail, and climb on it without anyone noticing, then you are probably best off booking an oceanview room without a veranda. There are plenty of places on the ship not in a stateroom where you can see outside.
Another option if your son won't be in the room without you is to book a Category 4 room that has a drop-down bed in front of the veranda door, ask the stateroom host to leave the bed down all day, and station someone in that bed at night who will wake up if someone comes near.