I think the real situation is far more complex than the simplicity that you are talking here. Remember, the US actively lists Pakistan as an ally in the war on terror and although Pakistan is a united entity, there is much in their territory that they don't have much influence over (especially terrorist elements). Also of note...the Western world seems to have a shorter view on history...if we go back to the 80's, the elements of Pakistan that we now list as terrorists were being trained and armed by the American military (Pakistani's will list this period of time as when the major criminal elements entered Pakistan and took up shop). More recently, the US has heavily funded the counter to the people that they were funding in the 80's. In any event, Pakistan is at the core of the Afghanistan mission and the larger 'war or terror' efforts.
As Henry Kissinger says "American has no permanent friends or enemies, only interests". In this case, it's interests are heavily stuck between long standing relations with India and it's newer counter-terrorism efforts within Pakistan. The newer relations with Pakistan aren't strong (read up on Salala if you need an example), and they are strained enough without an India-Pakistan escalation.
My guess on the US angle:
India - The US will welcome an increased Indian presence in the anti-terrorism spectrum, however they will want India to join the international forces (Nato) and warn against unilateral efforts (IE, India joins the NATO forces involved and allows NATO to make the calls where to attack...it will discourage India from making strikes on Pakistani territory).
Pakistan - The US will try to highlight the elements making the terror attacks as separate interests from Pakistan and pressure Pakistan to control this element (not unlike what's currently going on with the Afghan / Pakistan border). Of course, this makes Pakistan look weak (like it can't control it's own people) and will put the US and Pakistan at odds once again. The US will put pressure on Pakistan to allow NATO onto it's territory to address these terrorist activities, and the political dance continues.
I'm not sure if I agree with your take on China's stance on India...as much as India stands to lose here, China stands to loose huge as well. In 2008 the two leaders of India and China set a goal to increase trade between the two nations from 74 billion to around 100 billion...heavily imbalanced favoring China of course. This does include defence and military cooperation as well. This trade relation is starting to approach the US - China volumes and it's not much of a stretch to suggest that India - China trade could easily become the largest trade volumes between two countries. As much as it's easy to say that China is Pakistan's 'natural ally', China stands to devastate it's economy in telling India to 'face the consequences'. Why would China risk an economic dive bomb for Pakistan?
I'd suggest that China would also adopt a similar approach as the US, except be more likely to exert some influence over Pakistan. They will likely try to highlight the 'big terrorist activity' in India comes from elements outside of the Pakistani government as well.
Although China and US have very different reasons, diffusing a pakistani-india conflict is in both of their interests and I'd think you'd actually see some cooperation between the US and China in attempting to diffuse this conflict.