E.
T.:
Like someone else said - you have got to be completely honest with them. For two reasons: 1 - as you said, they have been good to you. and 2 - you don't want to burn this bridge.
Although it may be your "dream job", you may end up with a "nightmare boss" and you certainly don't want to burn the bridge with your former company if you find out later that this new job was not all it was cracked up to be.
Sorry, I am not trying to be a downer about it. I've just seen this happen with an old associate. She left on bad terms and now wants to come back but she didn't just burn this bridge, she CHARRED it to a crisp! Her personnel file has her marked as ineligible for rehire.
Be honest and say that this opportunity just landed in your lap and that if they were in your shoes, they'd do the same. Just make sure to let them know how much you appreciat them and make yourself available after your departure should they need help training a new person or whatever. In the end, you have to pursure what's in yours and your family's best intertest. If they're as wonderful as you say they are, then they'll understand. I just recently transitioned into a new position myself and had to leave a group of wonderful folks I've worked with for 10 years. I told them that the decision is bitter sweet and after much soul searching my decision was made in the interest of my family and their well being. I've made myself available for training the new folks and and have helped them when they had questions. No hard feelings seemed to have come out of this and they understood and would have done the same had they been in my shoes (my pay was significantly raised). Hope this helps! Good luck!