So… Amy just met herself as a kid…how exactly is she able to touch her?
A couple of different reasons, actually.
A lot of people tend to bring up Rose holding herself as an infant to explain away how these sort of paradoxes should bring more drastic results, but you have to remember one particular thing about that day: there was already a larger paradox at work from Rose trying to save her father’s life.
The Reapers were already there; that’s where the possibility of Rose touching her past self became dangerous. The most that it would have done normally is release pent-up temporal energy into the air— like the spark from my sonic screwdrivers, except less sparking and more… the air around them shifting a little. Prolonged physical exposure isn’t recommended, but it isn’t always dangerous.
You also have to realise that Amy was different from Amelia. Amelia had grown up in a parallel universe of sorts where stars had never existed; Amy was from her original timeline. They technically weren’t the same person anymore, kind of like how Mickey and Ricky could touch each other without any problems in… Pete’s universe.
I hope that makes sense. You have to understand that the universe is full of paradoxes; constant ones, shifting time and space. Reality is built on them. The universe is vast and chaotic and oftentimes doesn’t make any sense- but that’s also what makes it wonderful.