My brother has passed away. He was single and had no children. In his will, he left a small sum of money to my children. The notary informed us that the legal heirs must carry out the bequest. (...)

(...) How does this work for my sister who is under simple guardianship ? 

The guardianship allows your sister to perform all the acts that the guardian could do without judicial authorization (Article 467, paragraph 1 of the Civil Code), which includes the execution of bequests (Article 504 of the Civil Code and Annex 1 of Decree 2008-1484 of December 22, 2008).