A divorced couple were fighting over who would get custody of their child for the upcoming Pesach holiday.

This case is not like some other cases where we heard arguments based on one parent being religious and the other not. In this case both are religious - both are Haredi. And both the mother and the father want the child for the Pesach seder.

Why this was not worked out in the custody schedule I do not know, but it seems to not have been, and the beis din says their decision includes the considerations of custody arrangements (details of which are not mentioned) and the fact that the mother left the home, along with the following...

The beis din in Yerusahlayim decided the cased based on the mitzva of "vhigadta lbincha" - the father has the main part of the mitzva of retelling the story of thee exodus on the night of Pesach to his children, and this is not the obligation of the mother.

They go into great detail regarding the importance of the mitzva of every father telling the story of the exodus over to his children and concludes based on that and the fact that the father in this case says explicitly that he wants his son with him in order to be able to fulfill this mitzva properly, they are granting the custody for the first night of Pesach, with the seder, to the father, and the mother will get the child for the last holiday of Pesach. They do define the mother's part in the mitzva of retelling the story of the exodus as being very important, specifically in regards to transmitting her faith to her children, and specifically in the preparations in ridding the house of chametz and the preparations for the holiday in advance of the holiday including the cooking and the cooking amazing foods...
source: Behadrei

note: I only reviewed the psak briefly.. for more details see the behadrei article linked above.

Without knowing specific details of the arguments and of the custody arrangements and just assuming objectively equal arguments, I do find the decision based on the mitzva to be insulting and even degrading to the mother. She has to lose out being with her son because her ex-husband's mitzva is deemed more important? Don't we say in Pirkei Avos that we should not weigh mitzvas against each other as we do not know the reward for any specific miztva and do not know which is really more important than the other?  She gets the mitzva of cleaning and he gets the mitzva of sitting back and being with his son? Since when is the mitzva of ridding the house of chametz only on her? does a single mother not have a mitzva of retelling the story of the exodus to her children?