Can you share your child theme code for better understand and also it might be possible there is problem in your theme.
Check this link theme logo not changing in magento-2 custom-theme
In your admin open the theme configurations select your desired theme "STORES > Configuration > Design > Header (Click Choose file and select your logo)" and add your logo file and save your theme.
(OR)
Open your theme directory and check the following file "default.xml" is updated as below listed
app/design/frontend/vendor_Name/theme_Nmae/Magento_Theme/layout/default.xml
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd"> <body> <referenceBlock name="logo"> <arguments> <argument name="logo_file" xsi:type="string">images/my_logo.png</argument> <argument name="logo_img_width" xsi:type="number">300</argument> <argument name="logo_img_height" xsi:type="number">300</argument> </arguments> </referenceBlock> </body> </page>
In your admin open the theme configurations select your desired theme "STORES > Configuration > Design > Header (Click Choose file and select your logo)" and add your logo file and save your theme.
Open your theme directory and check the following file "default.xml" is updated as below listed
app/design/frontend/vendor_Name/theme_Nmae/Magento_Theme/layout/default.xml
<?xml version="1.0" encoding="UTF-8"?> <page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd"> <body> <referenceBlock name="logo"> <arguments> <argument name="logo_file" xsi:type="string">images/logo_new.png</argument> <argument name="logo_img_width" xsi:type="number">300</argument> <argument name="logo_img_height" xsi:type="number">300</argument> </arguments> </referenceBlock> </body> </page>