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, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
It appears - Allaah knows best – that it is permissible to hold a celebration on the occasion of a child completing the recitation of the Quran or memorizing it as a way of showing gratitude to Allaah for this bounty, provided that such a celebration is free from extravagance and showing off. Indeed, some righteous predecessors did this.
However, this should not be taken as a permanent habit so that people would not think that it is a Sunnah; rather, this should be done sometimes and not done at some other times.
Finally, it is better for whoever wishes to hold a celebration or slaughter (a sacrificial animal) on this occasion to remind the people [guests] that this is not a Sunnah but it is done solely for expressing their gratitude to Allaah.
Allaah Knows best.