You can find lots of nested repeaters examples[^] out there. Basically, in the ItemDataBound event hander of the parent repeater, you can access the child repeater, populate the datasource based on the current item, then bind it to the child repeater.
Basically, the Page_Load of the web user control does not run until the Page_Load of the web page instance has finished. And it does not make much sense when you want to change the event order in the life cycle. Can you be more specific about what you might want to do?
I have a datagrid with clickable headers and also a select all checkbox in the header of the datagrid. If i make the text headers of the datagrid clickable then the select all checkbox is not visible and if the select all checkbox is made visible then the text headers are no longer clickable. I want the text headers as well as the select all checkbox to be clickable at the same time.