First of all let me thank you for your response.
I had considered asking the petshop whether they would be interested to sell some of the offspring in the shop. One of the employees there keeps axolotls himself and gave us good advice when we first noticed the little ones. I would feel fine with leaving the babies there. I am sure they would be taken good care of.
Then and again, the petshop is part of a large chain of petshops here in Belgium and I do wonder whether it would be the employee's call to accept our axolotls' offspring. I do not know about the company policy.
However I will ask him the next time we go there, because the little ones are getting bigger all the time and I would not want to experience the offspring having offspring of their own.
I guess the prospect of having even more offspring has made me realize that contacting the petshop is the best idea. 
| 
