Inheritance law for surviving spouses and inheritance tax for children

Questioner

My father recently passed away, and my mother inherits half of the estate plus a child's share. She retains the usufruct of the remaining child's shares. Do the children have to pay inheritance tax immediately, or only when they actually receive their child's share? I understand there are various ways to circumvent this problem, including having the mother pay the inheritance tax. But if you follow the law without a notary and without any arrangements, how does that affect the children's inheritance tax? Yours sincerely,

Questioner

As a supplement/PS If my mother retains the usufruct of my child's share, does that child's share count for tax purposes when determining the amount of my assets?