How to insert into specific child table from portal

hi all ,

i have a doctype called x and inside this doctype i have a child table called y , in x doctype i have information like user id , email .

i need to push this doctype to the portal , and i need to get the doctype based on user session info ,
and i need this user to insert some information into the child table for his session info .

please let me know if you need more explanation !

Create a WebForm with Login for this Doctype