Definitely try to get in touch with the owners directly by mail or phone. May be able to dig up a phone number if you have the full name and city.

I would also have the tenants send a letter to the property management company about dangerous fence, liability, worried about injury to children, doesn’t comply with the lease, etc. The prop management co has no incentive, reason, or responsibility to respond to you at all. However, they have to respond to the tenants and should have some sense of duty to pass along that kind of letter to the owners.

If the owners hear from you on one hand and from the tenants on the other, maybe they’ll realize it is important.