10mth old Pup biting/snapping at 2.2yo Ds

[Claire2009]

She's a Jack Russell x Border Collie & always been pretty good, but lately when Ds is playing about, or if she's just lying on the floor & Ds goes to see her, she's snapping at him.
She hasn't drawn blood, but is leaving scratch like marks on his arm, just now she done it and he walked off holding his arm with a "Ouch" type look on his face.

I told the pup off, said "NO" in a firm voice and put her out, is that the right thing to do?
I've no experience of this, but I don't want a nasty dog around a 2 & 3yo.

are the teeth still small and sharp? We are going to get a puppy because of this and DS will learn without being hurt not to tormant the dog.

But dogs are heirachical - they need to be put in their place in the pecking order. If the dog things it is dominant over your DS then that is dangerous. Have you trained it, read any books on training?

No she has her adult teeth now, all the puppy teeth are gone.

I think that is what it is, the dominance/pecking order thing going on, she is trained to stay/heal/sit/paw/lie down etc and is generally very good.

It's making her realise she is not dominant over Ds..

By walking away he is giving her the signal that if she nips he leaves her alone. what you need to do is to teach him recognise when the dog wants to play and when she doesn't and don't annoy her. If she nips anyway tell him to squeal like another dog would and ignore her for a while. sounds silly but it does work. also make sure she gets plenty of exersize (AT LEAST an hour a day) as border collie x jrt sounds like a VERY lively combination! also, is she being socialized?