Thank you for your question
My name is Clare
I shall do my best to help you but I need some further information first
How old are the chidlren and how is their time shared between you
How often to they go to him mother's and for how long?
Why has his mother had to have them on those nights?
You certainly do not "owe" him for days that you have had them because he was away.
You are also entitled to know where the children are at night - it is a simple courtesy to ensure that if there is a disaster somewhere you know whether or not you should be worried.
Equally children are not packages.
I am sure that they enjoy spending the occasional night with their Grandmother but if your ex is routinely not able to have them then you are correct - they should be at their other home.
Your starting point is to discuss this with your ex to agree a parenting plan which deals with all of this using Family Mediation
If that fails then you may need sto apply for a Child Arrangement Order dealing with this point - although it is to be hoped that this will not be necessary
You can find more information on the website
I hope that this is of assistance - please ask if you need further details
Sadly that is not unusual - but you have to have attended a Mediation Information and Assessment meeting prior to making an applictaion to the court
He will not be at the first appointment and you can ask the mediators if they do Shuttle mediation so that you do not have to meet