A “Pacte de Famille” is not a deed per se (it is a generic term which can mean different estate planning options) so it would be useful to know which deed the notaire advised (maybe a donation-partage?). The choice of the appropriate solution depends on your circumstances and objectives. The adapted deed should not be a complex operation if you wish to solve a simple issue. Although I would not be certain without all relevant details on the matter, it seems that your children will sign a "renonciation anticipée à l'action en réduction" (effectively a relinquishment to their share upon the first death). The procedure is quite heavy so make sure this is absolutely necessary and no other "lighter" option would be available.
A notaire must provide a detailed quote at the outset and will usually request a provision for a slightly higher amount so he can be sure that all fees and taxes will be covered.
Legally, children cannot be disinherited but it is possible to delay the transfer to the second death. Children can also be disinherited as a result of certain estate planning measures.
I hope this helps.