I don't think you can change the goal posts on people who have already planned a family based on certain expectations of support, if and when needed.
But what might work is something that will come into force for the current crop of 13 year olds and younger, when they start having children. Something they are made fully aware is coming.
Rather than stick a figure on the number of children perhaps a flat rate for families, couples and single people is the same way a salary or a wage works.
You income when employed doesn't go up as you expand your family, it stays the same and you find ways to make it stretch further, perhaps benefits could follow the same model. There is no automatic right to benefits in Italy. It will depend on how much you have paid into a company scheme, or how much a government programme assigns, it will be time limited and it is not ajusted according to how many children you have.
While the negative birth rate here might not be without its own downside, there is a very conservative approach to family planning, and there certainly isn't such a huge number of teenage girls getting pregnant, despite a very poor sex education programme. I think a lack of government willingness to pick up the pieces if it all goes bent, knowing you will have to burden your extended family with your choices when in need, curtails people's willingness to base their family size on income during the good times and they tend rather to plan on what they can cope with when times are not so good.
So perhaps a flat rate form of support for families would be a compromise between the "based on family size" that the UK has now, and the "fuck all" you get over here that would allow for a positive birth rate while not being seen to encourage people to have more children than they can realistically afford to support, in bad times as well as good ?