my carrier (Spectrum mobile) does not support apple family setup. can I get a iwatch only cellular plan from a different carrier and then still be able to manage my kids phone from my spectrum iphone? thanks

That may be possible. From this support document, Set up Apple Watch for a family member - Apple Support Read the following:

If your iPhone cellular provider supports Family Setup, you can add the watch to your plan during this step.

If your carrier doesn't support Family Setup, or you'd like to use a different carrier, select Not Now.* You can choose an alternative carrier from this list of supported carriers and contact them directly to set up your cellular plan. Use the link to check, as it is going to depend on an alternate carrier that does support Family Setup.