Securing kid's bike inside the trailer while rolling

We used to keep them inside, laying on the area where the lowered table is.
Now, we have the Mount-n-Lock heavy-duty bumper. With it, I have a cargo tray, the spare tire and the bike rack on the back. It is a replacement to the standard 4" thin-walled, badly-welded square tube bumper than came with the rig.
We went about 6,000 miles last summer with nary a problem. Kept the bikes (especially when wet and muddy) which made Mrs. Shotgunner very happy. :)