Skyray,
Random is (or at least was) one of the criteria. There are many other things which could flag you as a selectee. Regarding your son... Drug smuglers have used children and babies as moles for years under the asumption that customs officials won't search the kids as thoroughly as an adult. If small children are excluded from the selection criteria then that leaves a very big whole in the process which could easily be exploited.
BikerMark,
The TSA does not require that you leave your checked luggage unlocked. They suggest that you do and tell you that *IF* a locked bag must be opened for inspection (most aren't) they will break the lock to open it. The decision to lock is completely up to the passenger. This is clearly explained on the TSA web site.
At many airports the checked luggage is scanned while in site of the passenger. In those cases you can lock the case and standby to open it if it needs to be opened or leave it open and ask them to lock it when they finish. There is also a new lock on the market that the TSA can open without breaking. A cheaper option is to "lock" it with nylon zip ties. If they need to inspect they just cut them off.
Radeng,
An open-jaw routing, or even a one-way ticket, does not guarentee selection. I have a number of one-way tickets in December and January. On the first one I was a selectee but not on the rest. I can only guess that the computer recongized the repeating pattern and took that into account.