Try Foothills Pediatrics. Dr Christensen, he has an office in Lafayette, off S Boulder Road and one in Longmont. My kids LOVE HIM! I will warn you the first time they let you go back but do encourage you to let the children go back themselves on future visits. I have to say it does work better, my son wouldn't sit for xrays when I was there, but when I sent him back without me and with his sister he did great for the staff!!! Kids tend to act up more or be more uncooperative when parents are present is the reasoning behind this. My daughter who is almost seven loves going back without me and my son too, not one problem.
They are very child friendly offices, where all the kids can see each other, you can peek in the window to see your son, they have video games and all that good stuff. All the staff there is super with kids. My daughter had to have a cavity filled this past summer (and I thought it would be traumatic) and they were great with her and it was a positive experience. I have taken my kids there since they were 18 mos old. I had to switch dentists recently (much to my kids protest) as they weren't a primary on our insurance anymore and it was costing me more. I highly recommend them though. They take pic of your child with a Good Healthy Teeth certificate too.