There are some free and low cost options here: https://www.opendns.com/home-internet-security/
You enroll for free or paid and then tell it your public IP or IP block to filter against.
To ensure users can’t bypass this DNS that you would serve via DHCP presumably, you could create firewall rules denying all DNS port 53/UDP except OpenDNS/Umbrella and that should do a good job complying with blocking adult content for a library.
My generally understanding for the laws around this in the US is that you make a reasonable attempt such as this and are not liable for reporting or instances where people find a way around it.
- Ex community all-star (⌐⊙_⊙)