RDU's Observation Park is a fun place for kids to go and watch planes land and take off. There is an observation deck, a little park with picnic tables, a mini runway and a sandbox play area.
Free parking, enter from RDU International Airport, between Terminal A and C, turn right into long term parking, then follow sigs to Observation Park.
Also, General Aviation Terminal is an indoor alternative. Home of Crosswinds Cafe and great views.