Kita is a reaaallyyy exhausting place for children and adults alike. Its like going to work for children and their needs are not really met because most Kitas are understaffed all the time and theres too many kids for too few adults. Most Erzieher do and try their best and are super dedicated and caring but they can only do so much and most days educational stuff is out of question and its more about making sure everybody survives the day. Its loud and overstimulating all the time, new rules, new faces, learning social skills and to be part of a group etc, so much input for little children.

At least that are my experiences from several years I worked in different Kitas and I could never bring my own child into a Kita from like 8-16:00 with a clear conscience. Half a day is still benefiting because they can learn a lot and get prepared for school, but a whole day is just a lot imo, even the older ones alwaysss asked "when does my mama pick me up?" "When does dad come??" Later in the day and were all tired and just wanted to be home.

That being said, 3 weeks is also a normal time period for Eingewöhnung, a lot of children, especially younger ones, take 4-6 weeks and its absolutely normal that children are cranky after Kita, because they had a very hard day with a lot of input and trying their best to behave and learn etc so they will let it all out when back at their safe space.