It is best to wait till you receive your bike to order 3rd party parts.
Your rear rack is rated to support 25kgs, or just over 55lbs. We cannot verify if they will work with your specific child seat, but there is a Ride1UP owners group on Facebook for owners that share their favorite after-market accessories.
The 500 and 700 Series rear racks use 16mm rack tubing diameter, and the 700 series has 10mm tube diameter for the pannier mounts.
The thread attachments for the rear racks (as pictured below) on all models are M5 threads.
Here are some of the common carriers used:
1) Front Basket or carrier:
Available in many types and sizes. Check the specifics to make sure it will work with your Ride1UP model. Most are a generic fit and work without any changes needed.
2) Cargo Trailer:
We sell one of these which is meant more for cargo, but can be used to haul your precious goods as well!
3) Rack Carrier:
There are many types and variations here. Here is an article on the rack attachment points and specifics on our bikes -https://ride1up.freshdesk.com/support/solutions/articles/65000167577-is-the-rear-rack-compatible-with-a-child-seat-
4) Top tube carrier:
These type of carriers should work with our bikes as well. Some edits may be needed and it is best to wait till you receive your bike to decide which seat will be the best fit for your bike. Unfortunately our staff will not be able to provide specific links.