Auggie usually loves to hear his parents talk about what he was like when he was a baby. It makes him feel safe and secure. Sometimes he just wants to curl up into a tiny little ball and let his parents hug him and kiss him all over.
There's no doubt about it; Auggie misses being a baby. The reason is that back then, he didn't know anything. By this, he presumably means that he didn't know about how cruel the world can be, especially for someone like himself, a young boy with a serious facial deformity. As a baby, he didn't have to worry about any of this; he could simply lie back and be coddled and fussed over by his loving parents.
On this particular occasion, however, Auggie doesn't care much for his mom's reminiscing. It's because she's trying to convince him to go to school and he doesn't want to. She tells him about how she took a tour of the science lab and how she and the other parents were shown a cute little chick hatching out of its egg. That's when she compares the cuteness of the chick with how cure little baby Auggie used to be. Normally, Auggie would feel all nice and warm inside at hearing this, but as he knows that his mom's trying to get him to go to a school he doesn't want to attend, he's not buying it.