I found a pea sized lump on the back my 1 year olds head. I can roll it a little like a cyst. It feels superficial to me, but should I be worried?

Probably lymph node. If there are no other symptoms, probably no need to be worried. Has there been a scratch or rash on the child's head or neck? Often these lumps turn out to be enlarged lymph nodes, part of the immune system. However, there is always a small chance they could be a sign of something more serious, so have your doctor take a look if it does not go away in a week or two.
BENIGN lymph node. A 'pea' size lump on the back of the head is probably a benign reactive lymph node. Lymph nodes are 'part of the immune system' and are there to help fight colds and other infections. A small (sebaceous) cyst is also possible. You should get seen if the lump grows, gets red/painful or if you are concerned. Hope the little one is better!
Simple vs serious. Our bodies always have things that grow and change. Skin, fat, muscle, lymph nodes, bones, can change shape, get larger, more prominent due to many factors. The vast majority are not cancer but simply unusual growths like fatty tumors, cysts, skin tags, moles, etc. Most cause no symptoms and can be watched. If they cause symptoms or change colors, bleed, bruise or hurt, then see your doctor.