We were with our kids who are teenagers on this tour in Vilnius http://www.baltictours.com/info/tours-in-vilnius/ and they liked it so much. What to do with kids it depends on how long you will spend in Lithuania. If you have time, you can go in Vilnius, then in Kuanas by train, it's very convienient. Then you can spend few days by the seaside in Klaipeda or Nida.
