It takes strength to recognize that it's best to stop. Personally, for me, the goal is just to keep riding, in any form, until I drop dead.
So, we went to the LBS. Clueless teen worker brings out the pinkest toddler sized helmet for my granddaughter. Both DH and I said, at the same time, "NO PINK." There were several other options, but, she is so little, her head is just a bit too small for the smallest toddler size. We got home and ordered an infant large, online. It's a cool blue color, with lots of illustrations.
She went nuts in the LBS, running around, saying "bicycle" about every 2 seconds.