I am honestly not one to bash my exes, none of them really. I have always been able to move on without too much nonsense. This includes with my ex husband, who also happens to be the father of my older 3 children.
The reason I am giving a little back story before jumping right in is because I want to show that there is not bad blood between us and that we can talk like adults. He contacts me when he needs a sitter for his other children, he has 1 older children (prior to our relationship) and 2 younger (after our relationship) and all are on good talking and communicating terms. Anyone that knows me personally knows the kids all hang out, we do holidays and birthdays, Fathers Day, etc together or try to at least involve all of the extended family to some extent.
If you are divorced, or separated, or for whatever reason apart, and I suppose this works for parents in general....please learn to use some form of basic communication with or to your children.
If they are too young and do not have access to their own phones or the internet, just contact the other parent. Regardless of any ill will you may have, you communicating to your child will go far in the long run.
If your child is old enough to communicate on their own please, just take, I don't know, 7-10 seconds to send a text, maybe 2 or 3 minutes to send an email, that says your plans are cancelled for whatever reason.
There are many reasons - sick, forgot, running late (even for days), work, just can not make it- regardless the reason, just communicate.
Let me tell you that what you have left is the other parent trying to make excuses for you and some of them are not as forgiving and nice when they speak about your lack of following through. Even if those reasons are true and legit they are not coming from the correct source so nothing being said really matters.
I am, by no means, parent of the year. We, as a family, have plenty of ups and downs, but I do think that I can say disappointing our children for whatever reason should be avoid as much as possible. I understand things happen, parents work, they get sick, there are accidents, they simply forget and that is fine off and on. When it is consistent and there is no form of communication it leads to your children not feeling like they can trust you, that they can count on you. It breaks the relationship between you down.
So please, just communicate.