Use list in parent site in custom list lookup field

What you need to do is create a site column at the top level site (IT) which is a lookup to your XXX list, then in your sub site you add the site column into your custom list.

From the parent site (the top level site where the list resides)

Go to Site Actions > Site Settings

Under Galleries select Site Columns

Click Create at the top of the list

Give the column a title you will reference in your "child" lists

Select Lookup as the column type

Select the site's column group to categorize the new column

Select the list to lookup from the "Get information from" dropdown

Select the column of the lookup list you want to reference as the key in your "child" lists

(I selected "Title (linked to item)" so that you can easily reference the parent item from your child list item)

Click OK

From the sub-site (site that you need to create the child list)

From the All Items list vies, select Settings > List Settings

At the end of the Columns list select Add from existing site columns

You can filter the selections by selecting the group you associated the parent column with from the dropdown

Select the site column you created above and click the Add button

Click OK and there you have it.