Apparently it was true in seasons 1, 2 & 3 but then there were casting issues. It might be true in season 4, definitely isn't true in season 5. In seasons 6 & 7 there is a variable truth that may make it true in seasons 1 & 3, 2 & 3, 1, 2 & 3 or just 1 on it's own. We now know for a fact that it's true in season 8, despite the tricky schedule negotiating period relating to an Oscar nominated film.
If you're trying to found out from the current release editions of the DVD's, official streams or rips thereof, the answer is held in the scenes with the actual children. Sometimes it's a digital insertion, sometimes you'll have to do some image manipulation like compressing the vertical shift or playing with the colour balance. In one instance there's a Holbien manipulation that'll have you printing out the picture and then looking at it from a funny angle. Quite a piece of genius that one and you'll know when it is because it completely ties in with that episode's plot. My favourite one though is when the third line of the second character in each scene contains a title in the actors filmography. The third letter of the second word in each of those lines makes an acrostic to spell the name of the Mother.
That's how I would have done it anyway.
