All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
The mosques are basically built for the prayer, mention of Allaah and recitation of the Quran, as the Prophet told a Bedouin. [Muslim] Therefore, in principle a mosque should be free from playing in it, but if such celebrations are restricted to the Muslims greeting and congratulating each other and the like, without the existence of any prohibition, like prohibited mixing between men and women, then it is permissible to organize such a celebration in the mosque.
It is also permissible for the celebration to include some play or fun that achieves a benefit, as in the story of the Ethiopians playing in the mosque during the time of the Prophet . Imaam An-Nawawi provided this incident as evidence for the permissibility of playing with weapons and any armaments in the mosque, and he said: "This also includes all the means that help in Jihaadd and other forms of righteousness."
However, if the mosque has a public court outside it or it is possible to rent a place in order to organize such a celebration, then it might be that this is better.
Allaah Knows best.