Plasma chains are child chains, not side chains.
"Plasma child chains and sidechains are not the same, in that a sidechain relies on its own validation process to determine the security of that chain, whereas a Plasma child chain may have its own validation process too (such as a PoS layer) but its security ultimately depends on the validation of its root chain (Ethereum). A very big difference, pointed out in the Plasma whitepaper" - source
"Plasma isn't a sidechain! Final security comes from the root chain, but the Plasma chain itself also has security properties that do not rely on trust of the root chain. Whereas sidechains (such as Blockstream's Liquid, or Loom's Dappchains) by definition must trust the main chain. The underlying design principles are different." - source
For more information about the Plasma framework, check out
For an easy to understand version of the Plasma whitepaper, check out this ELI5
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