I don't know, even 8yr old can make the link between it being freezing cold outside and the need to put clothes on a baby, especially outside!.
I can't make up mind about CM at all. I'm sure she knows the basics of taking care of a baby, I mean she knew and took steps to keep herself warm, and she had cash, they bought nappies and a dummy, a pushchair, twice by the sounds of it, but they couldn't nip into a charity shop a buy clothes, blankets, a coat? She knew the baby needed those things, it seems to me she just didn't care enough to get them.
They risked exposure to buy a meal in a restaurant, to buy supplies in argos and boots, to buy petrol (which I think was for a fire but not a cremation), they took taxis with the baby, so it wasn’t the risk of buying clothes for a baby that stopped her.
I don't know what the reason was, but I don't believe for a second it's because she didn't know better. JMO