If you have confirmation that the policy applies at AFKL operated lounges, even if you are on a non-AFKL flight, then you should file a complaint for sure.

My own understanding is that any Flying Blue related benefit would only apply if you are flying AF/KL marketed and SkyTeam operated. Essentially, you should have a AF/KL flight number. In your case, your flight number was a VN one, and technically, it would've been VN paying for your access as STE+, which might be why access was denied, because you were not treated as a FB member, but rather a STE+ member to whom the policy on unlimited children do not apply. Many years ago, at BKK, I tried to go into a different ST airline operated lounge than the one assigned by the airline itself (I think I wanted to use the AF lounge, instead of the one CZ assigned, because it was close to the gate). At the time there were a few ST operated lounges, and I was told clearly that I'd need to use the lounge that is assigned because the airline pays for that one, but was let in as an exception since I only had to be there for like 30-40 mins. It's not the same thing, but I suspect a similar thing, because each ST airline pays for lounge access on its own and they applies STE+ access rules. The AF lounge in this case was acting as a STE+ lounge for VN, rather than a FB lounge for AF/KL marketed flights.

But if you have seen a policy to be otherwise, and the PSL confirmed it, you should 100% raise a complaint. In fact, it would have been worth asking the PSL agent to file a complaint over the phone already. I'd probably file this complaint under the "problem at the airport" category.