The offence is very minor, but still has serious effects.
It depends on who the Garda issue the vetting report to. If they issue it to your husband, then he can choose not to release it if it contains the drug offence, and make his excuses as to why he cannot train the kids. The garda may not include the drugs reference, as they may see that it doesn't seem to be relevant.
If the garda intend to issue it directly to the team, then that's a problem. I find it hard to believe that the garda would issue the report directly to the team. There would be privacy issues, and the issue of genuine mistakes.. if the report was issued with mistakes, .. it wouldn't be possible to put the genie back in the bottle, and so the garda should really check the details before releasing it to third parties.
Would there be a record somewhere of his offence? There might not be,.. at least not a computerised record,.. in which case perhaps the police will miss it. They do ask for all addresses, even foreign ones, .. so they do vet people quite effectively.
The US imigration will likely take a stronger stance, and it's likely he could never enter the US. Of course the US drug laws are too strong in my opinion, and adolescent experimentation in the US can have lifelong consequences.