Children up to the age of 18 qualify for Kindergeld. Are your children still in education, they even qualify longer up to the age of 25. Currently the monthly payments are 219 € for your first and your second child, 225 € for your third child and 250 € for every child thereafter. Do you already receive child benefits from another country and the amounts are smaller? In this case you would receive the difference from the German state.
If you would like to apply for Kindergeld you need to apply for it as soon as you qualify for Kindergeld as you could apply just for 6 months backwards after the submission of the application. Here is the link for the application form: www.arbeitsagentur.de.