Hi MrsWibbly,
My best guess is the game registered the mom to be at work, even though she was still painting at home. Therefore the game registered the baby to go to daycare. With the baby registered to be at daycare, dad wouldn't be allowed to attend to baby. As the game thinks baby is at daycare.
I'm not sure it's quite a bug, but definitely a game glitch.
I tried to search on this, and really didn't get anywhere with it. So if you find that it happens again, it might be worth reporting it to answers.ea.com forum using the link below.
Answer HQ > The Sims > The Sims 4 Bug Reports
http://answers.ea.com/t5/The-Sims-4-Bug-Reports/bd-p/The-Sims-4-BugsHope this helps, and Welcome to the Forum.
MrsFlynn