I don't believe you have to put that it is compliant, on the label. What you do need if it will be something used or marketed for a child 12 and under, is the FTC and CPSIA label. The only thing you need for proof, is a copy of the test results on file. You would then provide that if you needed to.
As far as the buyer, you can't say "CPSIA approved"...as they don't approve anyone, you could say "CPSIA Compliant" if I recall right. It's up to you though, if a customer wants proof ( would be weird) of being compliant, if you provide that. I don't think you HAVE TO give it to them though.