Dog jumping up

Question
Hi There,
I have a one year old mastiff that within the past couple months has been jumping up and using her paw on my six year old son and other children in our yard.  She only does this outside and when I am NOT present.  She has excellent house manners and outside manners with adults.

When the kids see her coming toward them they move their arms around alot or run-I believe that this behavior has caused her to think they are her play toys.  I have tried to tell the kids to ignore her like my 10 year old can do (he has no issues with her) but it is hard for them I guess.
We have a large piece of property that my dogs have run of and there are alot of children that enter my yard sometimes without my knowledge and sometimes to come and see if my boys can play.  I do not want to keep Lucy in the house all of the time, as she loves to be outside when the weather is nice (and so do my kids.  I am looking for some technique to teach her that behavior is not acceptable.  But the dilemma is that she will not do it when I (or any adult) is present.

Answer
Small children don't understand that by running away or getting anxious/excited it is only making the situation worse because the more they are running or hopping around the more excited and jumpy the dog is going to get. You must always be present when your dog is interacting with your children,especially small children, this is a big dog that could do alot of damage without intending to.Even the nicest of dogs can injur a child without intending any harm.
As for neighborhood children wandering into your yard, you need to find a section of the yard where the dog can be when it's nice but can't get to any kids that wander into your yard looking for yours. That is a huge safety and liability issue having the dog loose when small children frequently come into your yard. If Lucy were to bite someone's child or injur someone's child when you left her outside alone when you know children come into your yard, you would be liable for damages, especially with such a large breed dog and knowing that she is not careful around kids.
The best thing you can do is make sure you are always supervising when she is around your children since your smaller children aren't able to understand that they need to inore the behavior like your 10 year old does, they should never be alone with the dog.