Pagination with multiple groupings

I have a data model that looks like this:

Group 1
--Sub group1
----item1
----item2
----item3

--Sub group2
----item1
----item2
----item3

Group 2
--Sub group1
----item1
----item2
----item3

--Sub group2
----item1
----item2
----item3

There will be n number of each group and I would like to display this data on screen.

A requirement is to use paging but I am a little unsure of how to effectively page when there are these groupings involved. Ordinarily I would just retrieve the next n records and append to the existing set but will need something a little more complicated.

Does anyone have any experience doing something like this? I don't want to grab entire sub groups at a time as they may contain too many or too few items, and would like to be consistent by pulling the same number of items each time.