Can I take Kinder Happy Hippos though TSA (USA)?

There shouldn't be any issue with TSA, as they are concerned with the normal airport security question of dangerous items. TSA permits non-liquid chocolates and candy.

The issue with Kinder products is around Kinder Eggs. These contain toys that do not meet US safety standards for children under 3, and so are prohibited. You can read the FDA Import Alert for "Confectionery Products Containing Non-Nutritive Components"; you'll see it lists Kinder Surprise and other chocolate eggs specifically.

As far as I can tell, Kinder Happy Hippos do not contain toys (or, in the language of the government, "Non-Nutritive Components"), so this shouldn't be an issue.

You'll be asked to complete a customs declaration form when you enter the US, and one of those questions asks if you're bringing any kind of food. You may have some additional questions as a result, but my experience is that you'll usually be waved through pretty quickly if you just explain you have prepackaged candy.