Is this the biggest thing? I don't know, it's right up there. The thing: being pregnant and/or postpartum often interferes significantly with the care of other children. This makes us very uneasy and unhappy. I don't know that there's any getting around it for the at-home mom. Having spent so much time convincing ourselves of the importance of being constantly available to our children, it doesn't feel right to be incapacitated and unavailable.
But wait--incapacitation isn't a choice. Would you feel guilty about breaking your leg or having migraines? Think like a Christian, self. Pregnancy is not a choice (or more accurately, an attempted non-veto) for a married Christian woman. It is, where God ordains it, a way of life.
So where God has ordained it, it is a way of life for children (even very young ones) to sometimes have a mom who just can't be catching toads and pushing strollers and handcrafting ravioli and maybe even keeping up with lessons for a few months. Remember: all that stuff we've read about mothering was written well after birth control and small families became the norm; even before homeschooling as we know it today looked like it does today (whatever that is). Children are Christians too. We must stop thinking of them as our personal projects and measures of our own success. They must live with us even as we must live with them, and we must all live for now on this old earth.
Which means, children also must learn to bear their crosses. It hurts to watch, but they must. This is one of those crosses, and really a more gentle one than we might think when we're mucking through it. Most of the time, Mom is still there even if she isn't attending them as closely as usual. It doesn't last forever; she gets better and comes back. They gain the experience of doing more things for themselves and each other. I'd be willing to bet that it seems a lot worse to us than it does to them.
Excursus: I remember my mom recovering from surgery when I was nine only because it was fun that grandma came to stay with us. To my parents I'm sure it was a calamity. I didn't even think about it until I was writing this. And what do I remember about my twin brothers being born weeks before my fourth birthday (with a not-yet-two-year-old sister between us)? How fun it was talking to mom in the middle of the night while she nursed the babies--which my older children and I now do.
It is not given to us to engineer our children's lives for their ease. Ease is as dangerous to children as it is to adults. It is given to us to guide them in Christian sacrifice and model for them Christian sanctity. When we are down and out as the result of a new baby (born or unborn), the best thing we can do for our older kids is not drag ourselves into the playroom and sob through a puzzle during that precious 63 minutes while the newborn sleeps, or teach the three-year-old to count by tallying Mom's vomiting sessions. The best thing we can do for them is bear our crosses in humility, explain the situation to them in a simple way that does not vilify the baby, and spend time with them as we are able while entrusting their care to God, who sends his Spirit (and maybe also Dad or Grandma or an uncle or a friend . . . or maybe not) to comfort all of his children, including the pregnant and postpartum ones.
We cannot keep bad or hard things from happening to our children. Our work is to help them through the bad and hard things that happen in life, and also to help them learn to distinguish what is bad from what is hard--a skill in which the world is thoroughly bankrupt.