I'd like to filter the available child record types a user can create based on the parent record type, any ideas?
As techtrekker mentioned, I'd suggest creating a validation rule. I'm not aware of a way to filter immediately and would be interested to know if there's a better solution.
As a quick test I created a validation rule on the child object, similar to the following:
In this example, if the child record type's Id is not equal to '012E00000001lB5' when the parent record type Id is '012E00000001lBA', the child record fails to save.
If you're going to be using this validation rule in multiple org's, a better way may be to create a formula field on both the parent and child objects that reference the record's RecordType.Name. Then you could adjust your validation rule to something like this: