Well, let's see...Easter is April 12. so their dates fall a week after Easter and beyond. Typically the two weeks surrounding Easter are pretty busy in Orlando (through April 19th), but drop off after that.
So, how much they cover depends a great deal on the choices they make.
If they go to Universal at the start of their trip, they will experience near peak attraction waits. If they go to Universal (and WDW ) towards the end of their trip, they will see significantly lower crowds. If they are only going to US one day, they should go at the end of their visit, or at least mid visit.
Whichever WDW park they choose the 1st day or two (MK?) - they should consider spending first days doing FP and shorter wait rides between FP. Then maybe return to MK (or whichever was 1st park) on a slower day when r-ride waits will be shorter.
If they can use peripheral hours, they will cover MUCH MORE than they will if they arrive after rope drop, or don't use the latest PM hours. (all parks) To cover the most rides all week, make sure to use the last 3 hours of park hours each day, especially Universal.
How much they cover will also depend a little on how many slow meals and breaks they take, and how willing they are to eat at slightly off hours. An easy one: eat lunch BEFORE 11:45am or AFTER 1:30pm. If you try to eat at noon over Easter week- you will spend extra time waiting in line, or waiting for your table. It isn't that rides are slow that hour, so much as noon meals take more time.
As of today, the new Hagrid's ride is still in flux. It will likely be fine, but popular come next spring. Hard to say how much time they will have to give for it. If they get in line before rope drop, they will have a far shorter wait then if they get in line mid-day.
Otherwise, even without EP, they should be able to cover most of both parks in one day, again - if they are up early and stay late. We usually skip most shows, and the VERY wet rides. but we also re-ride our favorites using the Single rider lines. they might not be comfortable using SR if she has a 9 year old. (maybe for re-rides if the daughter has her own phone?) The wet rides are VERY wet. In spring, we happily sip them, and might be closed anyway (if cool weather). Jurassic Park ride is showing its age, but Pteranodon Flyers are fun.
Getting a wand via Olivander's show, shopping, and using a wand will also take a up a good chunk of time, if they do those. Wand spells are SUPER fun! they take alittle practice, so don't get frustrated. to work, the wand tip MUST be pointed at the window. Move the wand, according to the spell directions, but don't swing the tip.
Only show outside of HP, that we tend to see is the Animal Planet one. That one is fun. We also stop by the fun fountain (near Sinbad) and go in the really cool art gift shop (both near HP), but don't see the Sinbad show most of the time. Poseidon's Fury is a walk through attraction., Pretty neat, but pretty intense (scary), and takes a bit of time. We don't always see it. Seussland is lots of fun.
If they want to see the Hogwart's light show, they need to be in the area early. It gets full. Once full, nobody gets into Hogsmeade until it is over.
they might also have to skip the Minions to fit in everything. OR - during busty times- there's a short queue for people who are willing to sit in the stationary seats. You still get 3D view, your seat just doesn't move. It is a 10min wait instead of 120min, so we often choose that when Minions has a long wait.
Otherwise, I dunno, they should consider staying 1 night at Royal Pacific to get EP for 2 days, and spend 2 days at US and have time to enjoy owning a wand. Universal can mostly be done (minus Hagrids, very wet rides, and regular Minions), eating all QS in one day, but is well worth 2-3. Plus, Royal Pacific is very nice! they will love the frequent boats and short walk to the parks. We have AP's so we frequently hop via the train ride. Another reason 2 day hoppers make good sense.
Eating at US is also a little different from WDW. the two HP quick serves have very good food, if you are open to the options there. (British themed food). At 3 Brooms, we love the pulled chicken salad (not listed on big electronic menu). In Diagon Alley, we like Bangers and Mash, and the hard ice cream is super! (clotted cream and apple crumble!) some of the other in-park QS are not nearly as good. Some are not good at all.
If they want super inexpensive food- Citywalk has: pizza, Burger King Whopper Bar, Bread Box (sandwiches), Menchie's fro-yo, Moe's, Panda Express, Starbuck's and at dinner a walk up sushi window (limited menu). We like Breadbox, but avoid the grilled sandwiches (they are a bit greasy). For Breakfast, Royal Pacific has a quick grab and go area in the AM. There's also a ton of nearby offsite options if they'll have a car.
If they want TS, US has some great options. You don't have book far in advance either. Among top choices are : Anotjitos, new Bigfire, Cowfish sushi/burgers, Toothsome Chocolate Emporium.
WE also really like the food court at the Aventura hotel, which is also QS and VERY millennial. (fresh, customized options) More places to eat at the hotels, too.
The other big thing to know about US- you do well to carry little in the parks. Backpacks have to be stowed at many rides, takes time, and now big lockers cost a little $. Only small lockers are free.
It is also a good idea to put at least one of their park tickets in a lanyard with a (water-proof) plastic ID carrier if they have one. I think you can get them super cheap online. You open lockers by scanning your ticket, and can keep the lanyard while you ride. US sells US themed ones, but they are nearly free elsewhere.
