Wondering about a bike helmet for toddler?

Hi, we're buying my daughter a tricycle for her 2nd birthday in a few days. Since we live in upstate NY, she won't be able to ride it outdoors until May or so. I am on the fence about whether to get a helmet now or wait until spring time and take her with me to get one to make sure it fits properly. I'm thinking she doesn't need one indoors (we have carpet) but another part of me doesn't want to fight the battle of her not understanding why she has to wear one when it comes time to ride outside. Maybe by then, she'll be old enough to understand that when she rides outside, she has to wear one, since we will keep the tricycle outside once the weather breaks. Any ideas?