I have a 4 year old and a 7 year old. They both love TOS (favorites by their own choice are Arena, Corbomite, and Tribbles). 7 year old is starting to watch some TNG with me, but you need to pick carefully. Some can be rather boring for a kid that age.
Also: my 7 year old has seen all the TOS movies. He loves TMP and TSFS. The 4year old has only seen TMP and asks for it rather frequently.
I first watched TMP as a re-run one afternoon when i was 8 or 9 and remember loving it. Although i got a bit scared by the transporter accident. Now if i try and watch TMP I will without a doubt fall asleep.