Hi,
The type of credits awarded for periods when your wife was looking after your children, are not required for the purposes of an E104/S041 certificate, and that will be the reason that they have not been included on her certificate.
If your wife requires proof of how many qualifying years she has, and details of the information held on her National Insurance (NICs) records, she can request a statement of NICs.
To do this she will need to complete a CA3916 application form which can be found here -
Get a PDU1 certificate or a National Insurance contributions statement
Thank you.