My aunt is going to lend me some money. We have planned for me to repay it in monthly installments over a period of 5 years. A loan deed will be regularized by a notary. If she dies before this term (...)

do her children have the right to demand that I pay them the outstanding amount in one go?

No. On the death of a person, the rights, and shares that he held during his lifetime are automatically transmitted to his heirs (art. 724 C. civ.). The Court of Cassation deduces from this, consistently, that the contracts entered by the deceased are transmitted to their heirs, under the same conditions (not. Civ. 1e, 2 June 1992 and Civ. 1e, 5 Dec. 2012).