In late April, when the final child is delivered, Little Clumsy is asleep.
For infants, they're shockingly similar- Clumsy hadn't said any actual words at his first word ceremony, simply tumbling off the moonlit stump and going "oops!", so Papa is worried he may be developmentally delayed; and Little Brainy will get that same worry in a few days when Papa realizes this young child barely reacts to anything, except to burst into tears when he bumps into something. (Everysmurf will realize decades later that Brainy is blind.)
But the next day, when the babies are playing, Clumsy trips over Brainy, causing the younger smurfling to cry. In an attempt to cheer him up, Clumsy gives Brainy his favorite plushie. Neither of them will remember this, and an adult Brainy isn't sure why he keeps a ragged old toy around.
They don't interact much as toddlers. Brainy gets his glasses, and Clumsy is mostly sheltered as Papa tries to figure out what's wrong with him. Once Brainy learns to read, the first smurfling to do so, he becomes a know-it-all. Clumsy, meanwhile, can't tell his As from his Zs. The two of them have nothing in common.