If taxpayer money from any govt agcy (i.e. state, federal, city, county, etc.)
is being used in any way to help support this parade is it a violation of law
to prevent other groups from participating in the parade?

For example, is taxpayer money being used for the police officers to be present at the parade?
Or is "The Days of '47 Parade" organization paying for the police and/or any other govt. employees
who help to make the parade possible?

Days of 47 is not a religious event. Recently some places call it a heritage event. It simply celebrates the first settlers that happen to be Mormons.

By extension, Thanksgiving could be a religious event since it celebrates the first settlers in Plymouth that happen to be Puritans.

Many cities and states have a heritage or founders day celebration.

only coincidently for security, clean up.

some cities/counties require payment of fees for those so other taxpayers don't subsidize them.

The children's parade is a week before. I was in a college marching band that performed in it. They opened it up with a prayer. I hung out with nonmormons in the band. We all kept our eyes opened and were silently laughing about it.