The security of Bitcoin itself/the blockchain won't change at all if the functionality for sidechains is implemented.
However, the sidechains themselves (and the Bitcoins pegged to them) will only be as secure or vulnerable as their implementation and usage is. The security of a sidechain, is as far as I'm aware, the exact same as it would be if the sidechain was an independent altcoin.
It is correct on first order that Bitcoins is isolated from bugs in sidechains. However, a good case can be made that sidechains could lead to more mining centralization, which is always the elephant in the room with Bitcoin.
Wouldn't it be the exact same centralization situation if the sidechains in question instead were developed as altcoins? The altcoins/sidecoins that use merge-mining may lead to centralization while the ones that do seperate mining probably won't?
The problem is that bitcoin mining has very slim margins now, and if sidechains happen, "mining difficulty" will go even hight and mining sidechains will be pretty much be mandatory for ROI.
However, mining many sidechains requires significant computing resources.
However, the sidechains themselves (and the Bitcoins pegged to them) will only be as secure or vulnerable as their implementation and usage is. The security of a sidechain, is as far as I'm aware, the exact same as it would be if the sidechain was an independent altcoin.