Language Capability of a Three and a Half Year Old

Since my son was about two and a half I've noticed how his vocabulary and sentence structures have been improving. He's my only child so far (one on the way) but I think it is general behaviour for most children to have jumps in their language skills. Over night they suddenly use more words and use longer and more complex sentences. This really makes me think. I want any story I write to stretch the child that is being read to. I'm now a tad stricken with doubt that I'm dumbing my stories down. Certainly I'll review my second book with this in mind. I might then overreact and publish something way too complex for the third. Anyway, it's useful to keep the child's capability in mind, and to aim to pitch a story a few months ahead of where the child should be.