I'm severely clinically depressed along with anxiety and other symptoms and I have a 5 year old daughter. I try my best, but there's no way I could possibly pretend all the time. And nap time doesn't last forever, of course. We've told her that sometimes people's brains get sick just like their bodies do, since your brain is a part of your body. We also tell her that brains getting sick can make you feel sad, even without a reason. I think we started telling her that around three years old, and once I started needed more treatments that she needed to come to the waiting room with us for, we just told her the doctor was giving me a treatment to make my brain feel better. She does get concerned still if I start crying but she doesn't seem scared and she's a very happy child in general. We want to do whatever we can to make her feel comfortable with emotions, both hers and other people's, and we hope if she has problems ever that knowing my history will help her come to us.
I'm so sorry you're going through this, but as long as you're talking to your kids in an age appropriate way, I don't think you should worry about it hurting them. Calling your midwife sounds like a great idea. Something I've done when I'm struggling to contact someone is to ask my husband to make the call for me. It feels silly to me, but it gets the job done. Also, are you in therapy? Your therapist has probably encountered this situation before and probably could help you work out what you want to tell your kids.
Also, when I was pregnant, I went cold turkey off all of my meds and refused any treatment apart from therapy, and, to put it mildly, it was horrific. I wish I hadn't done that and I don't know what you're doing now or willing to consider, but there are options of meds and treatments for pregnant women. I hope I'm not being pushy with that, but I especially hate to see someone suffer like that.