Hi @Alexa.Ki!
Can I ask why you are opting to use the Conditional ternary operator ( ? ) ? While it would compile, it has a different function than you may have intended if it is supposed to read exactly like the above code.
Whereas if you used the logical operator && and treated the second variable as a bool like
if( : : IsValid(ChildMesh) && (IsValid(ChildMesh->SkeletalMesh))
would that not be appropriate for what you are attempting to do?
Any more relevant information to what you are trying to achieve would definitely help solve your mystery and I hope this helps!