My mother had serious heart disease since before I was born. Trust me, your 7 year old knows something is wrong and/or different about you vs. other moms even if she doesn’t remember your trip to the ER. Kids pick up on this.
The problem is, if you don’t say anything the kid’s imagination may take over and paint scary pictures for her. I also agree that a Formal Talk is not the way to go, either. Manda Jo’s suggestion is spot on - bring it up casually. “Mommy has to get her MS medication. Do you know what MS is, honey? It’s a disease, but the medicine keeps it from being bad and helps keep mommy well.” A kid that age wants to know that, even if mom is sick things are under control. Even if you go to the ER, the approach with the kid should be “mommy’s MS is bad right now, so she’s going to the ER because that’s where she needs to be to get better.”
Don’t try to pretend everything is perfectly OK, because she’ll know it isn’t. You need to keep the information age appropriate, but she’s old enough to understand something of what’s happening, and as she gets older so will her ability to understand. I recall as a kid there was a tendency to keep me entirely out of the loop. After mom’s first heart surgery everyone else filed into the room to see mom but I was not allowed to - and trust me, not knowing conjured up FAR worse scenes in my mind than how a person actually looks after such surgery. At least in my case, it would have been kinder to let me see mom for a couple minutes than to leave me wondering for years what horrific scene was occurring in mom’s hospital room.
Of course, this varies with kids - I have a far higher tolerance for that sort of thing than one of my sisters, who was probably traumatized by being dragged into that room when really she wasn’t prepared for it and didn’t want to go. You’ll have to consider the personality and traits of each child, rather use a blanket statement of “Y is OK at X age”. At a certain point it becomes appropriate for a child to visit the sickbed of someone in the hospital, but I’d suggest giving the kid an out: “You can come in, but I just want to let you know, if you start to feel upset or uncomfortable it’s OK to leave. Some people find this more upsetting than others. How you feel is OK, as long as you’re honest about it. It’s kind to visit people in the hospital, but if you start getting upset you need to politely leave because the sick person needs their energy to get better and it would be unfair to add to their burdens.” There were times I went with everyone else to the hospital and spend more time in the waiting room than visiting mom because on that particular day I didn’t have as much cope as another. Kids need to know there is nothing wrong with that, because too often they try to fulfill the expectations of others in these matters (especially if they’re the sort that think good behavior on their part makes mom better and bad behavior on their part makes her worse - it’s not rational, but it’s not uncommon at certain ages for that sort of notion to take hold).
Another thing to remember is that, even as an adult, how people react to illness in others varies enormously. I have a sister who can’t deal with medical issues in our parents at all, and another whose a hospice director with nothing but dying patients mostly our parents’ age. When mom was in her final months Dr. Sister was heavily involved in the medical stuff, but my other sister couldn’t handle being in the same room as mom - she helped out with house cleaning, grocery shopping, and running errands so the rest of us had more time and energy for the medical crap and it was totally OK with me. Sure it would have been nice having more help cleaning up mom, but at least I didn’t have to deal with mom AND the house! Working within our limitations kept us all sane and the arguing to a minimum.
I guess what I’m saying is that, I’m sorry you have a serious chronic illness like MS, but it’s part of your life now and you can’t, in fairness, insulate your kids from that fact. They’ll know you have a problem. If you keep them informed they won’t have unnecessary fears, but they don’t need to know ALL the details.