What's the best way to seal a small leak on a sprinkler pump inlet PVC pipe where a siphoning system has been tapped?
Tighten the joint  slightly. 

Pipe threads are tapered - the PTFE thread tape (or pipe dope - I'm currently more of a fan of PTFE pipe dope than of PTFE thread tape, though I used to be a big fan of the tape as opposed to old-style dope) provides a seal for the "helical leak path" (the fact that the female threads are a bit sharper than the top of the male threads) and some lubricity for easier tightening. 

A slow drip type leak is often just a 1/4 turn from not leaking. 

If you take it apart, you need to clean off the remains of the old tape and apply new tape (one reason I now prefer the modern dope - you can just add a bit more, with no need to clean off the old, and you don't have all the tape shreds to deal with.) Excessive thickness of tape won't help the seal - more is not better.