In our case, each "indivisaire" is absolutely free to sell his/her part of the property without the consent of the others. The sale will be valid but "inopposable" to any other dissenting "indivisaire" until the "partage" is efficient. In this respect you can refer to the following decision (Cour de cassation 15 juin 1994) Détail d'une jurisprudence judiciaire
Any member of the familly agreeing to sell to the grandaughter could do it for their share (or simply draft a "promesse de vente" for their share as the Cour de cassation also agrees that such a contract is perfectly valid Détail d'une jurisprudence judiciaire).
At the end of the day the aunt would only be able to control the price of her own share (if any) and the share of her children until they reach 18. The fact that the sale would not be "opposable" to the aunt should not be considered as a real trouble because it will be "opposable" to the rest of the world. The aunt would remain in position to ask "partage" to the Court but she would only be able to recup "her" share, eventually at market price (with the advice of an expert)...