Re: What would you do?
It is indeed hard to give an opinion about this without knowing details. Coming to the wrong conclusions and making wrong assumptions is easy to do.
You stated that your relationship has cooled down to the point you believe it has no heartbeat anymore and that the only thing preventing you from taking off is your children's happiness. I think it's great of you to think it over really well before making a decision, even posting it here for us strangers to read. I do think you deserve your own happiness, even if that means a tough time for your children. But leaving and moving somewhere else would indeed have a huge impact on them, no doubt about it. That's why I think that should be the last option or your list of measures to take. (I'm assuming that "I'm in a relationship with a man" means you're not married, so sorry upfront if I assumed wrongly.)
I don't know how you got into this relationship, but it is kind of odd for something that started with sparks flying allover the place to end up being something where romance is nowhere to be seen in just 3 years. Did you really love him when you got into this (love as in love and attraction,) or did you get together because (say) you enjoyed his companionship more than being alone? This question might sound a little harsh, but it needs to be asked in order to know if there is anything to be revived in your relationship.
You didn't tell me much about your man apart from him being a hard worker, someone who rarely raises his voice and that your son is very taken with him. How does your daughter feel about him? I'm trying to determine your children's current happiness, with him part of the family.
Does he know about how you feel? Do you two talk about your problems or are you more of the "ignore until it can't be ignored anymore" kind? You don't have to let your kids in on it, but voicing your issues and offering him a chance to voice his can resolve a lot of things. He must've noticed something's off for your sex life to have become non-existent. Did you tell him his "moves" didn't do it for you? I don't know why you mentioned his age, so I feel compelled to tell you that a man of his age can still have a very active sex life without needing v1agra ;-)
You said he is a very hard worker. A lot of stress often results in a diminished sex drive and lack of romance. Could his job be making him unhappy? If he's struck a chore careerwise and became distant in the relationship, leaving you bored and unhappy, it might be the right time for a new careermove. Also, would experimenting with new things be an option? A holiday to a far destination neither of you have seen before, or small things like surprising him when he's taking a routine shower...
Reading your post I had to make an assumption and say you're probably not someone who is quick to complain. He might be afraid to ask.
If you haven't already, I suggest you have a really good talk with him about this. You deserve your happiness as much as everyone else in the family. Still, if leaving brings happiness only to one out of 4, I'd suggest leaving only if you feel you (and he) have done everything in your power to save this relationship.
That's my opinion. Based on assumptions based on nothing. Still, I hope it makes some sense and I hope you aren't rolling your eyes at me right now thinking "duh, I already thought of that."