Do you have a partner? If so now would be a good time for them to take the day off. You are going to have many days where this happens over the next year and as far as possible if there are two of you working you should split the days off. On this occasion where it's important for you to be seen as reliable, I'd say it's their turn. (Apologies if you're a single parent, in which case this doesn't help).
I'm due back to work on Monday but today my baby has been ill, vomiting and fever. Probably picked it up from the nursery settling in sessions. What is the best thing to do? The nursery won't take a sick baby (and I wouldn't want to be away for the first time when he's not well) but it's way too late to delay my return to work even though I'm going back before the full 12 months as I've been officially back 2 weeks already on annual leave.