My kids visited with a friend this afternoon who learned this evening her aunt has covid. she tested negative, but is quarantined for exposure, and is apparently symptomatic as of this evening. should i quarantine my children?

It sounds as though you are describing an exposure to an exposure. That carries with it a microscopic risk of transmission. You should be guided by local public health expertise, however I would not require it. Be sure to encourage all involved to get vaccinated as the best protection against severe disease.

I would not be concerned unless the aunt lives in the same space as your child's friend. Getting the children immunized adds another layer of protection against COVID and adds to your peace of mind. Best wishes.