Are Chinese clone brands considered to be safe for children?
Is there some kind of official source regarding the safety of bricks
made by brands other than LEGO®?

As far as I am aware, no.  At least not specifically for construction brick toys.

Some brands (Xingbao, CaDa, Mould King, Happy Build, ...) have some
really nice sets (original designs of course) and are officially
available for purchase in the EU/Germany so they need to have a CE
marking and possibly passed other tests like TÜV.

The CE marking indicates a product has passed health, safety, and environmental protection standards for sale within the European Economic Area. The individual tests vary between industries but are not necessarily a certification of quality. The CE marking is also found on products sold outside the EEA that have been manufactured to EEA standards.
LEGO products adhere to the EU Toy Safety Directive and the US Consumer Product Safety Improvement Act. The approval process involves 'simulating things like children biting, dropping, and stepping on LEGO elements to ensure they don’t break'*
In regards to the concern of chemical safety of LEGO clones, CE is a good indicator that a product passes certain safety criteria, there are of course other certification marks covering different regions.
If a LEGO clone has no credible certification mark does that mean it is unsafe? We simply do not know. The effort to test the safety of all these uncertified brands is too great. Parts vary widely in their colour (each individual colour has a unique chemical composition) as well as material (whether they are hard plastics, soft plastics, transparent, opaque or even glitter plastics in some cases).
You could always clean your new bricks as a precaution, if you suspect that there might be any surface level contamination, such as oil or plastic residue used in the manufacturing process. This will not make a difference to any toxic chemicals in the composition of the bricks.
In summary, if you do not see a certification marking that you can trust, there is no guarantee of safety.