I doubt that the OP is suggesting that people on benefits should not be able to have more than 2 children, or that they should abort any further pregnancies, etc. People responding to the OP like that are being a bit over-dramatic or sensationalising.
It is also a bit daft to say that children don't cost much. If that was the case, why were people so worked up about the possible loss of child benefit or Frank Field's proposal that it stop at 13? I read nothing on MN but a list of how expensive teenagers are when that was bandied about. So it's daft to claim that additional children are not more expensive.
I think (hope) that s/he is making a (valid, up to a point) observation that benefits are - to a certain degree - elastic, namely, your income is adjusted by the state to better fit your circumstances (whether this happens satisfactorily in practice is another matter).
On the other hand, it is a well observed fact (lots of MNers complain about it) that those earning money do not have "elastic" incomes (although CTC has recently transformed this for those on or below a certain income), so if they do have more children, they do not get any more money for those additional children; therefore, you frequently see people - even on reasonable salaries - saying they cannot afford more children.
Therefore, it is unsurprising that people imagine that those on benefits are "luckier" as their incomes grow with each child.
Structurally, it is true that this is a kind of "inequality" (that is, if you are in a certain frame of mind, you might feel that, as someone who earns a living, it is not fair that some people see their incomes increase with more children, while yours remains the same).
Now, this is obviously to ignore the fact that - for the overwhelming majority of claimants - life on benefits is very hard, and having children does not make it easier.
But, what this argument misses is the fact that - OBVIOUSLY - the state does not believe that the money it gives to benefit claimants is enough for people to live on if they have children/a family; so when they have a child, it hands out a little more, then another and a little more, and so on. It is a way of adjusting the barely-able-to-afford-to-live line. So for that reason, OP, YABU.