I'll try and keep the information neutral, including gender and which side of the argument I'm on.
Background: Couple married for 12 years, two children, separated a year ago after one party's infidelity. One parent has main residency, the other has the DC eow and a couple of times a week for tea.
Eldest DC has a birthday which falls on the day that the resident parent normally has the DC. This parent asks non-resident parent to tea to watch DC open presents and do the cake etc.
After this, the resident parent drops the DCs off at their grandparent's so they can visit someone they are in an early relationship with. The NR parent then calls the R parent to speak to the DC and at this point discovers that they have been left at the grandparent's house. The NR parent is very angry about this but the R parent maintains that the DC enjoy spending time with their grandparents and wanted to stay over that night. The NR parent says that if they'd known about these plans they would have had the DC even though they already have birthday plans for the next day with the DC.
So, oh wise MN jury, considering the facts:
Was it unreasonable for the DC to be left at their grandparents' on their birthday?