That's not how anything works. A knowledge gap doesn't warrant any conclusion you want.
Also most humans would say 1 but the truth is that you can't know. It's a stupid question getting sometimes stupid answer.
I am almost tired of people coming up with ridiculous "breaks" to feel better about themselves
Then surely the only "correct" answer is to say the second child has an indeterminate number of red balloons? Concluding it has 2 balloons is wrong imo, its an answer which is not supported by any of the information supplied in the question