After digging a bit deeper into plasma the following question came to my mind: can a TestNet - e.g. Rinkby or Ropsten be a SubChain in plasma? As far as I understand this might be possible and one could e.g. move tokens between TestNets and the Main-Net. But I would love to make sure If I understand this correctly.
As far as I understand, tokens can be moved between two blockchains that do not have any special relation, as long as they have smart contracts and thus can somehow verify state transitions in the other chain.
This is true for any two blockchains.
The only thing that makes the relationship between a parent and a child chain special is the circumstances under which the parent will declare its child as invalid. Since this does not require anything from the child chain, any (smart contract) chain can be a child chain of a plasma chain.
If you consider it further, there is also no real requirement for the parent chain, apart from some specific smart contract being present there that can verify state transitions in the child chain. So nothing stops you (apart from the gas requirements perhaps... ;-) ) from deploying a smart contract on Ropsten that makes the Mainnet a child chain of Ropsten...
Now the second question is how the parent chain deals with tokens coming from the child chain and vice-versa.