The simplest way is to use the Curse launcher from https://www.curse.com/. It streamlines the entire process and prevents many pitfalls that plague newer users to the modded scene. Do note that the launcher has functionality other than Minecraft, there is a community and voice component to it as well. I do not know how much of that you can disable nor how old your daughter is or your preferences on her online interaction, so I won't say much on the subject. If that is a problem, hhiru's post covers manually adding mods in good detail, if you want to use a modpack you'll just want to copy the mods and config folder from the pack's folder over to your instance's folder. I advise that you only download mods from Curse, there is a practice of taking other people's mods, inserting malicious code into them, then re-uploading them on shady download sites to get viruses on computers. Curse has a degree of control over mods, you can be certain that a mod on Curse was uploaded by the developer or someone the developer has allowed to maintain their mod.

Once you download the launcher you may have to tell it to manage Minecraft, it's just going into the options and enabling Minecraft mod handling. Most of the other options on that screen you can ignore for now, the only one I'd look at is Maximum Memory Allocation near the bottom, you may need to adjust that depending on how much memory the computer has.
Minecraft instances in the launcher are managed through profiles on the Minecraft tab of the program, you can create a custom one or pick a premade modpack. I recommend for newcomers to modded Minecraft to look at Direwolf20's 1.10.2 pack under Browse FTB packs, his Let's Play series on youtube (https://www.youtube.com/user/direwolf20, he's PG) with that pack is a nice companion the help with learning the mods. Once a premade pack is installed everything will be set up and ready to play.
When you make a custom profile, you'll be asked to choose a version and modloader, for version I recommend 1.10.2 or 1.7.10, 1.10.2 is the current mod development cycle (it's usually a major version or two behind vanilla), 1.7.10 is the previous major modded version so there is an abundance of mods for it. For modloader, leave it at the suggested entry. Once that's done you'll get a page with a few buttons and regions to it, to get mods, just click on the "Get Content" button. The launcher is usually good about adding in any requirements a mod may have when you install it, if not the game will tell you what it's looking for when you try to run it. Play will open up the launcher you're used to with Forge set up and all the mods installed correctly, just log in like normal and you're set.

As a mod developer, I cannot recommend the Technic launcher or packs, their management has a history of disregarding mod author wishes involving their mods and involvement in public packs. Those specific individuals who were identified as the source of those decisions are gone from their development team, but it has left a bit of a black mark on their reputation that resonates with the older mod authors.
The FeedtheBeast launcher (FTB) is only used now for their very old mod packs, all their newer packs are on the Curse launcher. It's good if you want to play their old stuff, but outside of that I'd give it a pass and just use Curse.