The thing is, school stuff such as the Nativity in the evening cannot be made compulsory, but they do rely on everyone joining in.
Schools put these events on for the enjoyment and education of the kids. They sometimes have them in the evening because otherwise working parents complains they miss out on everything...so they try to provide something for everyone. I'd imagine, the teachers could do without another evening in school or at the Church or wherever it is too.
If everyone just picks and chooses the events that are convenient for them and doesn't send their children in, it really does make putting on any kind of performance really difficult - it is about the bigger picture.
Families with several kids have to cope with the in-out, in-out impact. This really is the thin end of the wedge and the amount of things DD will be involved in over the next few years, which will involve taking the others in and out at often inconvenient times, will be significant. She will gain a lot from those experiences, even when they are sometimes inconvenient. The alternative is to deny her the experiences or not let her attend.
And I I know that in this particular case,mashed doesn't want to go. However, in my mind, that really isn't the issue, but is a bit convenient for the OP who may well have fuelled her lack of enthusiasm! The real issue here is that OP doesn't want to go and feels aggrieved about the whole thing - a festive event is being put on, that DD has been included in - it wasn't voluntary and everyone is expected to be there.......so she should be there.
OP, this stuff is going to come up time and time again. People do find ways to manage babies and tiny children in this situation and you can too. Even DHs with the most high powered jobs can get home on the odd occasion if given plenty of notice. Friends can help drop off and pick up. Babies can go out.
See this for what it is, a festive enjoyable event, that your DD will benefit from. Stop being a misery and seeing the school as trying to make your life awkward - they can't arrange everything to suit every individual, but are trying to put on some events which will be fun, but only work if the vast majority show the commitment needed. Make a plan, go, enjoy it.....and start planning for the future, because this is going to happen again and again and your DD will miss out if you are going to take this attitude long term.