Is it safe to fly if my child has a cold?

Probably. As long as your child doesn't complain of ear pain or have a significant fever it will probably be ok. If your child has a predisposition to ear infections or flud in his/her ears then i would recommend you have the ears evaluated by your pediatrician or ENT prior to flying.
Yes. If your child is old enough to tell you if anything hurts and he does not have a fever with a mild illness there really should be no problem with flying in most cases. If there are worsening symptoms he should be evaluated before your flight.