As-Salamu Alaykum wa Rahmatullahi wa Barakatuh.
In The Name of Allah, Most Gracious, Most Merciful.
All Praise and Thanks are due to Allah, and peace and blessings be upon His Messenger.
In response to this question, the European Council for Fatwa and Research (ECFR) issued the following Fatwa:
There is no problem with Muslim children purchasing these fireworks to display their joy and celebration for the occasion during Islamic festivities. However, because Allah (swt) forbids all excess, this must be done with moderation.
It is also illegal for Muslim children to play with these during Christian celebrations because it is considered a form of participation in their religious occasions, but they are permitted to do so at other times of the year.
Allah Almighty knows best.