The problem is herd immunity.
If those people are within our "herd" and one of their kids gets infected with ebola-marburg-plague-mumps-pox, then they become a disease transmission vector to the rest of the herd.
And in that case, everyone who comes in contact with them becomes exposed to it and those who have not yet been immunized against ebola-marburg-plague-mumps-pox, run the risk of getting infected.
Now, if that was polio, you get crippled and paralyzed.
If that was mumps, there is no treatment, you suffer and hopefully don't get an additional disease (30% testicular atrophy).
If that was measles, you get a 4 day whole body rash and a fever up to 104F (40C)
If that was rubella, it's similar to measles, but slightly less severe.
If that was varicella, well that's chicken pox and we mostly know what fun that's like. Plus possible scarring for life if the pix is severe + shingles later on in life.
We all know what little disease transmission factories kinder gardens are.
The parent who doesn't vaccinate their kids exposes everyone's kids (and their parents) to infection.