It is a solo game, start by enabling her
You have an opportunity to experiment since dealing with one player, who also happens to be a person very close to you, is an opportunity that seldom shows itself, in my experience. And I do dare say that it is an extensive experience.
The fact is: experience doesn't matter. The important part of D&D is not the destination, it is every part of the journey which brings you to keep playing (or not, since that's also a possibility).
Allowing players to play multiple characters and NPCs is double edged
It makes it easier for you as a GM to balance encounters since the rules are indeed written with some guidelines regarding group size and how to build encounters and other aspects.
But having to deal with multiple characters as a player, in an imaginary context that is imagined itself and described by someone else... it's a lot more complicated than focusing on just one character as a player.
I am NOT saying it is a mistake to give more "options" to your daughter, in the form of NPCs she can control, I'm saying this gives her a whole lot more information to process while also processing said information with a limited understanding of what you know as the GM. You know what is gonna happen, she's mostly guessing for most of it... the perspective is very different and you should keep that in mind.
Since you mention the problem as being her "forgetting to do stuff" I feel like your daugther's struggles with your game is due to information overload (or something similar).
There are multiple possible solutions
You could try to dumb down your story. She's a new player, ease her into it, simplify your story at least until she gets used to dealing with the actual mechanics of the game. It's about teaching her how to fish rather than giving her a fish... sort of.
You could also control "helpful" NPCs yourself. Make her part of a living world where she can rely on help, if she remembers that help has been offered to her character.
Remove the need for her to control multiple characters. Controlling a NPC is not much different than controlling a PC, except most NPCs are weak compared to your usual PC. I'm guessing some NPCs that she controls were actual PCs in the game at some point, which only adds to the 'information overload' she has to deal with as her options multiply.
Make the game less about fighting and more about 'a living world surrounding your daughter's character'. Ask your player about that, aka what she's looking for, what she might be struggling with, etc. and then keep that in mind.
Solo games are not harder, just different
They are different in the sense that usually, at least to my own (rare) experience with solo games, they focus more on character development than fights and the need to balance them differently.
They allow a GM to be generous and a player to go a bit more crazy with his character's development, since everything is focused on that one character. That gives a player a lot more opportunities to come up with complicated plans or go deeper with NPCs interaction without anyone (ie other players) losing patience. A side effect is it also enables a player to be more efficent with crazy plans, since there is no chance that any other player derails such plans in character as I feel is very often the case in "normal D&D groups dynamics", in my experience. Enable her to profit from that.
I remember running a solo game in Ravenloft
With one of my more "powergamey" players in my childhood group. As a DM, I had always preferred more gritty realism and really shied away from allowing my players to become 'demi-gods', if you know what I mean.
After a few months, both my player and I realized that the game was going much slower than we'd like and when I asked my player about that, he told me something that I remember as meaning "It is so hard to do anything, I wonder if this is due to Ravenloft or the fact that you like giving me a hard time". That made me think and before the next game I asked him what he wanted for his character's story, what would his big crazy goal be ? He deadpan told me "To vanquish Strahd and take control of Barovia, of course !".
That took me out of my comfort zone as a DM, but I started thinkings of ways to challenge him while still enabling him to take the necessary steps for him to achieve his goal in the long term. That guy played with me for years in my "gritty realism" style when he liked being 'epic and powerful', he played out of his comfort zone for me for a long time and it had never occurred to me to do the same for him until he asked me ... today, 20 years later, we both still remember that game clearly. His vampire hunter eventually became a vampire himself, struggled to deal with that for a while until he found a way and the power he needed to kill Strahd. He conquered Barovia! We even kept this game going after the conquest, I had to invent rules for 'epic levels' in AD&D 2e. We eventually both got bored because hey, I mean, he really was too powerful at the end, but it was good fun getting there!
That's my example where being more permissive and paying more attention to all of my player's expectations created a funner game than it would have been, even if it got me out of 'my usual style' as a DM. In fact, that game made me enjoy DMing more 'high fantasy/epic' games in the future and I'll admit that my players seemed to enjoy that too! For years, we alternated between 'gritty realistic' and 'high fantasy' games.
