If you brought your child to Germany and would like for them to take up German citizenship, the German officials will require a renunciation certificate. Which you should apply for from the Kenyan Embassy in Berlin.
However, the Kenyan law DOES NOT allow children to renounce their citizenship.
According to the Kenya Citizenship and Immigration Act of 2011, only people older than the “maturity” age may apply for renunciation. Maturity age in Kenya is 18 years of age. In cases where someone younger than that applies for a renunciation, the Cabinet Secretary has a right to decline the application on the basis that the individual renouncing their citizenship doesn’t duly understand the implications of such renunciation.
If you are applying for German citizenship for your child who is still not 18, you still send in an application for renunciation and the Embassy issues an official statement which you present to the Ausländerbehörde. The child will usually have both citizenships.
In the current law, the child would receive a letter on their 18th birthday requiring they pick one before they turn 23, failure to which they automatically lost the German one. However, the coalition government has signed a new law that allows children born in Germany to keep both citizenships for life. Whether this will apply to children who take up German citizenship before they turn 18, I’m not sure.