I think your daughter may have shin splints. It is not serious at all. Shin splints can occur when someone is running around alot more than they had been and are a little heavier than they were when thye last ran. Atletes get them in training camp and preseason games.
You daughter is tall and consequently heavy for her age - certainly heavier than she was when she was last running around outside. She probably is playing/running more with the warmer weather lately and that running may be on the concrete playground at the day care. The harder surfaces outside compared to inside can also contribute to the shin splints.
Make sure her shoes are comfortable and the soles are padded. Forsake 'nice' shoes and keep her in sneakers. Rest is also something that would help but I know that would be difficult with a 3yo. You can also massage her legs right next to the shin bones with your thumbs - this with move the lactic acid out and bring in some blood to heal the minor damage.
Good luck!