TRACON FLM providing a skill check described a loss of separation event when a second departure overtook a preceding aircraft; the reporter listing both a failed hear back error and confused 'climb via' procedure/s as causal factors.

Narrative

I was the FLM performing a skill check on a developmental. We had just taken the position and had a few traffic situations that he had fallen behind on. Aircraft X; a Runway XX departure; was climbing on a vector/classic SID. Air Carrier Y was a [Runway] YYR departure that was on a RNAV SID. Both aircraft meet at a way point called ZZZZZ. Air Carrier Y was overtaking Aircraft X. Aircraft X was turned eastbound; climbed; and [then] stopped once we realized the overtake. Traffic was called to the trailing aircraft; Air Carrier Y; but he was already above Aircraft X and he couldn't see him. Loss of separation occurred. In addition; the trainee told Air Carrier Y to climb via the SID; which means to cross ZZZZZ below 7;000 FT. Air Carrier Y stated; 'Roger; cleared unrestricted to FL190.' Trainee did not catch the read back error; nor did I. We had a couple of difficult situations that we were dealing with at the time. Had Air Carrier Y stopped the climb at 7;000 FT; there would have been no loss of separation. I feel that these 'climb via' on the RNAV SIDs cause more confusion. Some aircraft are climbing to 7;000 FT only and some are climbing via the SID; this is confusing. If all aircraft came off climbing to the same altitude and let the Departure Controller climb them; traffic permitting; this [might] not have happened. Most of the pilots are extremely confused on these 'climb via' SIDs.