What an exciting time for you.
Since you asked about Lafayette, I'll confess I am a fan. I like the small town atmosphere of Erie, Lafayette as well as the Frederick-Firestone communities. Lafayette has a few nice restaurants of its own, but you'll be close enough to Louisville and Boulder for afternoon and weekend drives.
Another possibility: Have you heard that a company is planning to construct a monorail service between Longmont and Boulder? Longmont is approaching the size of Boulder, but it is more of a family-oriented community. Evidence? My three teenagers couldn't wait to get out of town when they started driving. But, I love Boulder for all it has to offer, too. (Actually, there is plenty for teens to do here.)
Erie has a plan for a large development working through its approval process, and it could become a much larger community. http://www.dailycamera.com/erie-news/ci_24269978/erie-oks-annexation-golden-run-but-obstacles-lie
Lafayette, which has Boulder Valley schools, plans to remain distinct from its close neighbor, Louisville. RTD says they will run light rail up through Louisville, but that won't happen for decades.
Longmont and Louisville have been working diligently to improve their old town centers. Both communities are encouraging art galleries, restaurants and small concerns to locate in their downtowns. Things turn busy on weekends. All of these communities serve an active lifestyle. Boulder has excellent schools, and Longmont's schools rate high, too.
There's more. Visit my blog for a large list of parks, recreation areas and trails. Some of these areas need work after the flood in September.
We did a lot of picnicking at Boulder Creek (behind the public library) when our kids were 4. (I had twins.) Dad would take the kids hiking on the trail up to Flagstaff. We took advantage of a lot of these parks and rec areas. Fun times.
New theaters are planned for Twin Peaks Mall in Longmont.
I could go on.
Best of luck to you.