A common injury for toddlers and pre-schoolers is "nursemaid's elbow" (also called subluxed radial head -- aren't you glad we call it nursemaid's elbow?). It typically occurs when the arm is tugged or pulled (lifting a child up by the hands or wrist, swinging them around by the hands or wrist, etc.). The child will have sudden pain in the elbow region. Often they will hold the arm to their side with their elbow bent, not wanting to move the arm. There is a terrific review article about it for parents here at Healthy Children. If you suspect your child has this condition, we want to see your child in the office or have them seen at Nationwide Children's Hospital. There are some children that have this happen once only. Other children have it happen a few times -- yet the orthopedic specialists will so often tell us that they will grow out of this once their elbow strengthens up over a few years. Discuss this with us if your child has this happens more than once.
One note: if I help a four year old up onto our exam tables here in the office, I always reach under their underarms to lift the child. When asked why, I always explain that I am not wanting to lift by the hands or wrist so that I do not cause a nursemaid's elbow. I encourage you to lift your young children by lifting under their underarms.