I am going to... this is going to sound bonkers and like ya right... but
There was (and not sure if it was ever fixed) an issue where accessing Child data via scoped down from Parent, would NOT cause an error but the data would be blank WHEN you tried to write out Gallery data, even though you could see it.
2 different possible solutions.
1. use an Alias for both the Parent and the Child, this works sometimes, cannot tell you why and why not
parentgallery.AllItems as Parent_Gallery and do the same for gallery, then access stuff using the alias
2. you have to add some ridiculous column and control into the parent.. like a text box and the text box actually represents some data in the child.. like count of rows under it, it doesn't even have to be visible, just there
So for kicks add a Label. in it put the count(child rows for this parent). Make it visible at first just to see if you see values. Then test it
And bingo.. .child data would start appearing
PS FYI for option 2. I noticed that if i did this AND used Aliases, it barely ever worked, so I took away the alias..
But :-( give them a try.