There are many sports clubs offering children and youth sports. Please ask for information at the local sport clubs.
An overview of sports and sports clubs in the state of Niedersachsen can be found here: https://lsbntweb.lsb-niedersachsen.de/extern/vsuche.osp
Special sports offers for people with a migration background or socially disadvantaged conditions can be found here: https://www.lsb-niedersachsen.de/lsb-themen/lsb-sportentwicklung/lsb-se-foerderprogramme/se-sportangebote-einzelmassnahme/