The government shouldn't have to mandate immunizations, but unfortunately we have turned "I have the right to be wrong" into a mantra in this country. There is absolutely no reason not to protect your kids' health and the health of the community by using the best modern technology to combat disease. If government mandate is the only way to make this actually happen, then I guess that's what we'll have to do.
The only liberty argument at stake here is really the liberty to be a bad parent. And we really shouldn't be going so far out of our way to protect bad parents.