What are some cons of genetically modifying children before birth for intelligence?

This is a very theoretical topic. We're not at the point where it is happening quite yet on a mass scale, so statistics will be very difficult to find, and estimates at best.

Some dangers of the practice would be the fact that it would (like many things) favour the rich. Being poor would subconsciously make people assume that since your parents didn't have the money to pay for your intelligence enhancement, there's no way you could naturally be as smart as somebody from a rich family.

The human brain is a very complex thing, even now not completely understood. There's a chance the procedure would have horrible repercussions in another part of the brain, repercussions we wouldn't learn about until, perhaps, the modifying had been going on for twenty years or more. There is no animal with a brain similar enough to humans to safely conclude from animal testing that there would be no danger to humans.

A comparatively smaller consequence, but if you think children are cruel to each other now, just imagine what it would be like where the poor kids are assumed to be physically stupider than the rich kids.

I saw this in a twilight zone episode. Some of the kids had side effects and turned into monsterous creatures. Don't know how true that could be, but it seems possible. I think it would be cool though, to pick hair colors and eye color and intelligence and all that. I suppose the richest peope would have the smartest kids, eh?

In the future, there could be a racial issue between "normally" developed people, and the ones who were genetically perfected.

It could even lead to genocide.

Picture another holocaust. Us "normal" people being the Jews, and the "perfect/intelligent" people being the Nazi Germans.

Utter hell.