The baby is definitely entitled to its own room and it says this in the Regs.
The size criteria refer to the number of “occupiers”. These are defined in Reg 13D(12) as all the people who occupy as their home the dwelling to which the claim relates.
Reg 13D(3) then allows a bedroom for a child, or two children who can be paired up. A “child” is defined in Reg 2 as a person aged under 16 – it does not say anything about the child being part of the claimant’s family, just that it is aged under 16.
Sounds like there is only one child in this household, so a room is allowed for that child. It does not matter whether the child belongs to the immediate family of the claimant or of the non-dep couple – either way, it is a child who occupies the dwelling as its home.