I wanted to add that I saw from your siggy that your boys are pretty much exactly the age my dd was when she started violin (she started right after her 5th b-day). My dd is not exactly the patient sit-still kind of kid, but she has done very well with violin. I think two factors that have really helped is that her teacher is experienced in teaching younger kids, and also that in the Suzuki method there is a large parental involvement.
I remember when I was still teaching someone asked me to teach his 3 yr old. I told him I didn't have any experience teaching a kid that young, but I would give it a try. I found it very difficult and I suggested that he needed to find a teacher who was a better fit. But I imagine the same girl would have done just fine with a Suzuki teacher. Keep searching until you find a teacher who is comfortable teaching younger kids (not just one who will grudgingly take a 5 yr old student).
SunshineJ - I don't think it's necessarily a "bad attitude" at play here, but more a lack of training on the part of the teachers. I know I was happy and willing to try teaching that 3 yr old girl, but I just didn't know how. Teaching a little kid is much different from teaching a school-aged kid, and takes a much different approach (depending on your methodology).