Lots of good advice in
this thread and in
this one.
Will you be alone with the two kids? If so, you will certainly have your hands full. Make sure the cabin crew are aware of your predicament, and speak to them early on about planning meals, etc - it's best if you feed the kids first, and then tackle your own meal later.
Since you rather foolishly didn't check with us before buying your ticket, there is little point in my main piece of advice, which is to start long-haul flights in the late afternoon or early evening. This gives the best chance of getting the kids to sleep according to their normal schedule, which gives you and your fellow passengers the best chance of some peace and quiet.
Once you're all settled on the long flight, it will probably be mostly OK. The worst part of the trip is likely to be the change at Heathrow.
Good luck!