@Mami99 - how long has your child's account been in suspended/nonpay status. If over 90 days, the account is closed and phone number is gone. But, since this is a card replacement, I suspect it has been that long.
If an EverSafe username (email) and password was not set up for this account yet, the system may act differently when the account is Active as opposed to Suspended (in nonpay status).
I like the idea trying @dust2dust 's suggestion by using the "Forgot email" function: https://eversafe.id.telus.com/user/forgotemail?locale=en&ForceAuth=true. Then check all possible emails it could be connected to (remember to check the spam folders).
But if this account was not registered to an email, and the account is Suspended, then you could activate it by making a payment by other methods to pay other than through My Account: https://www.publicmobile.ca/en/on/get-help/articles?q=vouchers
Locations to obtain some vouchers found here: https://publicmobile.ca/en/on/payment-voucher
Then, once active try the "Forgot email" again, to get an SMS to the phone.
Otherwise, see @Handy1 's post to submit a ticket to Public Mobile support staff to get access into your child's My Account.
