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?
It's a sidechain parameter; see sidechains.pdf section 6.1 (page 16, line 479-480).
6.1 Hashpower attack resistance
The main thrust of this paper surrounds two-way peg using SPV proofs, which are forgeable by a 51%-majority and blockable by however much hashpower is needed to build a sufficiently-long proof during the transfers contest period. (There is a tradeoff on this latter point if 33% hashpower can block a proof, then 67% is needed to successfully use a false one, and so on.)
You mean that a sidechain can have a completely different consensus system...
Yes, they can have that.
...where the larger chain is beign somehow decided by more than 66% of the total hashing power?
The sidechain's consensus is always a simple majority balance like Bitcoin (unless someone invents some other algorithm) - the 66% requirement applies only to transfers out of it.