I am so sorry to hear this
Your kids will come back to you, right now they probably dont even know whats going on and like you said, dad is making everything super fun, when they want a REAL parent, theyll ask for you.
As for him, milk him for all hes worth financially, take his cheating no good ass to court and get him for spousal maintenance, child maintenance and any other financial expenditure you can.
Make sure that at least if nothing else, you dont ever have to worry about money while your kids are growing up, that will let you focus on all the other important things.