I think that if you can not afford children you should wait until you can and if you unexpectedly fall pregnant there should be a system in place to help but it should not be continuous as some people rely on this and do not do anything to get themselves out of that place
What you're basically talking about makes sense, and it already happens in a good many countries around the world, where there isn't such a problem with the benefits system being abused to such an extent.
I was talking to a colleague last week who comes from an EU country where child poverty is targetted via the tax system, rather than as a point of need system.
I.e. instead of handing out childcare vouchers, tax credits, child benefit to parents as well as taxing them the same as any other working adult (which happens in the UK), they have a system whereby there is less emphasis on handouts but a huge tax break if you have children to support. Hey presto, you have a system where people have no real method of living on welfare long-term just because they have children (because they are not directly supported) but they do have a means by which each family is supported if there is a culture of working in the household by some means.
Clearly there are issues with the alternative system described above (e.g. what if someone genuinely can't find work for a long period of time and still has children to support?) but I think that balance of focusing on the elimination of child poverty only in cases where there is a work ethic (if able bodied, etc) is an interesting one.
It's the complete opposite of how we approach things in the UK, which is to say there is no natural "limit" to how many children a state-supported family has, because there is no natural tailoff to the income the family has... unlike working families.
I also found it interesting that there was such an outcry earlier this year (last year?) when the government pointed out this "no end in sight" situation (where there is no limit to how much in benefits a household can have) was unsustainable. I'm not a Tory voter, but I was surprised at the backlash.