As a secondary physics teacher I was surprised to hear that you had difficulties finding past papers (or at least some sample papers) with markschemes.
I always recommend going to the website of the relevant exam board and looking carefully in their resources section.
I had a quick look at the AQA website and found this link which has the specimen papers and markschemes.
www.sciencelab.org.uk/support/question-papers-mark-schemes.php
Since your child is taking the new 2011 science exams there are very few past papers available for the P1 module and even less (perhaps none) available for the P2 and P3 modules. These past papers are so recent that they will not generally be available from the exam boards except to teachers at schools to use as mock papers for the students.
Looking more closely at the AQA website, it appears that you can actually purchase past papers and markschemes from them before you can download them for free. You can see what is available for free and for purchase and when they are available at this link:
web.aqa.org.uk/pub_policies/whats_available.php
To purchase past papers use this link:
shop.aqa.org.uk/
Personally I think this is very cheeky of AQA. The exam board my school uses has several free past papers available to everyone already and does not require payment for early viewing.
Good luck, and I hope this was more helpful than confusing!