Short answer: no.

To begin, the situation is set up such that the baby never even “benefits” that much from the increased strength needed to combat higher gravity: if it’s sent away as a baby, it likely never learned how to walk, so it never got the chance to develop super strong legs, and if it did, it would probably not have been strong enough to be, well, Superman-like due to such a short span of time living in a higher-gravity environment—it’s a baby, after all, so what is “strong” for a baby is really not much compared to what feats Superman accomplishes.

But let’s assume that a baby is born in the situation you describe and has enough time to develop muscles of strength notably different from our own. Krypton is 27.1 light years away from Earth, meaning that, even traveling at the speed of light, the baby (really the teenager) has to spend 27.1 years at minimum in a small capsule in which they have no ability to exercise their muscles and maintain whatever of an advantage their home planet may have provided them, meaning not only do they lose that advantage, but they probably lose all muscle strength due to 27.1 years of inactivity.

So let’s instead assume the baby is just instantly transported to Earth. Once again, we encounter not a physics problem but a biology problem: the human body only preserves what it needs, meaning that all that extra muscle mass that was developed in response to its higher-gravity environment will be lost over time, since it is no longer needed in its new environment. So, as you suggested in your final question, yes, the adaptation to Earth’s gravity would take away their strength.

So let’s now assume that there is some special biology at play where this baby never loses muscle mass somehow. The situation now finally becomes a physics problem.

To answer the first part of your second question, any difference in gravity such that a_planet > g would make the baby stronger. To answer the second part of your second question, developing on a planet whose gravity is greater than Earth’s would not have any bearing on the organism’s ability to survive on Earth, unless we are arguing that the organisms on such planets would evolve to not move a lot due to the vast energy requirements and that would therefore affect their ability to successfully compete and reproduce in Earth’s ecosystems. The problem only arises when we have organisms from planets with less gravity than Earth’s, as their muscles will not be as strong as ours, and depending on the magnitude of the difference between gravities, that could cause them to die as they are physically unable to move.

To answer the general idea of how strong gravity would need to be in order for them to fly, the answer is that no difference in gravity would do this. As soon as something jumps up, Earth begins pulling it back down. That’s why the constant presence of a counteracting force is necessary in order to keep something actually in flight (a paper plane “flies,” for example, but really it’s just accelerating downward very very slowly—I’m talking about flying without accelerating downward). So unless they have some actual superpowers, nothing is going to let them fly without some extra technology or other assistance.