It may state in there something to the effect of making up or changing visitation "as agreed upon by both parties" --I doubt a judge will look favorably on him for being in another state visiting his girlfriend while conveniently forgetting it was his scheduled visitation time with his children.
Read over your paperwork and see what is in there about visitation "make ups". Either way, if it was his scheduled time and he didn't see them because he was in another state, I think that's pretty indefensible.
I also agree with keeping a log of EVERYTHING. Missed visits, late pick ups, early drop offs and any and every issue in between. It may not be admissible in court, but your attorney will look at it and you'll have everything conveniently written down to jog your memory if needed. Believe it or not, as angry as you are at the time things happen, you will tend to forget the details over time. Those details could end up being very important at a later date. Write it down - time, place, date, what was said, what was done. Skip nothing.