I've often wondered why their skills were always supposed to be so much better than Odo's. He had plenty of time to hone his appearance and since he could easily imitate other complex shapes it seems unrealistic that he never actually developed throughout the whole series.
Most of the "complex shapes" amounted to solid, unmoving stuff though - once he became a rock, or a metal frame, or a bag, he only had to hold the shape. Faces also have to move in certain ways - he could probably have made a human "mask", but it would have been revealed as such almost immediately outside of still photos/holos.
But by that time, he had been stimulated and educated. "Children of Time" comes not only a great many years after "The Search", but also after Odo's to-Solid-and-back experience, and after many interactions with skilled Founders.
Actually, I wonder why he never had surgery during his "solid" experience to look like a Bajoran (or human). Outside of the show, the DS9 companion made it clear that they thought he wouldn't come across as Odo any more if they lost "the face", but since the Founders left it that way as a punishment