This is a place where old clothes get collected also you can donate children clothes that you don’t use any more. The people in need will be very happy with your contribution, no matter how small it is. http://www.kledingbankarnhem-eo.nl
If you are looking for a second-hand shop, this might be the place for you: www.kinderkledingbeurs.nl
Here are listings of all the second-hand markets for children’s clothes and toys.
One of the largest second-hand shops is called 2Switch and it has 10 shops all over the region: https://2switch.nl/