After a two-hour meeting at Windy Hill on Tuesday night, parents were told that the AFL knew in February that AOD-9604 was not a prohibited substance.
Click here for our full coverage of the Essendon supplement scandal
Watson said parents were "surprised" to hear that and looked forward to the League's response.
Sports medicine specialist Andrew Garnham, a former member of an AFL Victoria Anti-Doping Tribunal, said he had asked ASADA in February 2013 about the status of AOD-9604.
Bombers' D-day on delay after club requests more time
Garnham said ASADA told him the anti-obesity drug, central to the supplements saga, was not a banned substance.
Speaking after the fifth meeting between the club's hierarchy and the players' parents, Watson said parents wanted to know more.
"I think parents were surprised to have heard that and I guess now it's for the AFL and ASADA to explain why they haven't given any more information [on what] they've had for that long period of time," Watson said.
"I haven't spoken to the players so I don't know if they're angry about that.
"That's up to [the AFL to explain]. I'm not going to speculate on that. I got in trouble last time I speculated about the AFL and the Commission so I think Andrew [Demetriou] will probably come out and explain that in great detail to everyone."
As Watson walked away from the media, he quipped, "I look forward to the explanation".
Earlier on Tuesday night, Garnham said he made the inquiries about AOD-9604 "so there was some clarification as to where the matter might be heading".
Garnham, who is currently working as a consultant for Essendon, has previously sat on a VFL anti-doping panel, which is an appointment made by the AFL.
"The advice that I had at that time was that AOD-9604 was considered under section S2 of the anti-doping code and was regarded as not prohibited," Garnham told Fox Footy's AFL 360.
"I'm aware that other people within the AFL had been in contact with ASADA. I'm not sure exactly what time period."
Asked if AOD-9604 was a prohibited substance according to ASADA when he inquired in February 2013, Garnham said: "At that point in time, no."
"There was some discussion with people in the AFL and also at that stage I'd been in contact with members of the Essendon Football Club," he added.
Tuesday night's meeting ran for just over two hours and was the first time the players' parents and partners were addressed by new chairman Paul Little.
Watson said he had been "reassured for a long time" by the club of the players' position in the supplements scandal.
Another parent said they felt relieved after the meeting but added they desperately wanted the saga to end for the sake of everyone involved.
They were addressed by medical experts, including Garnham, at the lengthy gathering.
Watson said fears the players' long-term health could be affected by the Bombers' supplement program had been allayed as a result.
"We had an independent medical expert there today who I think did a great job of explaining what he knew about those substances and allayed a lot of the fears a lot of the parents had," he said.
"The club have tried to have been as open as they possibly can right throughout this and that's what they were again tonight.
"Of all the substances that are known in this ASADA report, they gave a detailed outline as to what they were and the effects of those if there were any and those fears have been allayed."
He also said there were no substances that were still unknown "as to what they may or may not have been, or whether they were administered".
"Of all the information available to the club via the ASADA investigation, all those supplements have been cleared in terms of harmful effects on players," he said.
Watson recently said he believed the AFL had "conditioned" the public to expect the Bombers to be stripped of premiership points the week before the finals.
He said he didn't know where the club stood in terms of playing in September.
"From what I've heard, I don't know. I said at the time it was just they way I was reading the report and I can't say any more than that," he said.
"I haven't got any more than that. I haven't got any more inside information outside of my own understanding of the way it was being reported at that time."
Earlier on Tuesday, it was confirmed that the charges against Essendon and four key officials were unlikely to be resolved before the finals after the club sought an adjournment for Monday's AFL Commission hearing.
The club has followed James Hird, Mark Thompson, Danny Corcoran and Dr Bruce Reid in seeking a postponement of a full hearing of their charges arising from the AFL-ASADA joint investigation.
