Then assets within donkeycoin's blockchain are susceptible to reversal and/or oversight/control by the 51% attacker.
If the attacker achieves 66% (or whatever the configurable threshold is for the sidechain), then they can also begin to steal outside assets pegged into that blockchain.
The donkeycoin asset/coin itself is irrelevant to this, and may or may not exist.
Wait where did that 66% come from? You mean that a sidechain can have a completely different consensus system where the larger chain is beign somehow decided by more than 66% of the total hashing power?