OP, I am learning to ski as an adult. I think the advice above is good.
Here were my main learning points:
1. Layers, but not cotton and certainly not denim. Wool or synthetic wicking fabric. This was a good site:
https://newtoski.com/how-to-layer-for-skiing/
2. Most gear can be rented at the slope. I'd advise good clothes, including gloves and goggles. You can rent helmet, boots, skis, and poles, or bring your own. Rentals suffice for lessons.
3. It's hard to walk in the boots. There's a weird rocking movement to stairs, and they will tire you out more than you expect. Book extra time for this, both to get from place to place and to get to the lesson site.
4. There always seem to be discount shops near big ski towns. You can get things like good pants here, albeit worn ones, if you like.
5. Please expect to tip for lessons. It is appropriate. I've tried to book individual lessons but been rebooked for group lessons. Group lessons are useful. If you want private lessons, please speak to someone directly and confirm.
6. There will be lockers. They may take coins or bills or credit cards. Be ready for all of them, unless you know for sure. There's also the need to lock various things up over lunch sometimes, etc. Just have extra cash of various sorts as well as a card.
7. It is damn busy and confusing when you get there. I'd suggest giving yourself a couple hours to get oriented. Ski places with lessons tend to be really crowded with people who know exactly what they are doing and where they are going. I found it to be a real muddle at three different places (I ended 2019/started 2020 with the goal of driving out to try all the ski places in my area).
- If you buy tickets online, you have to go someplace to pick them up on site. It may not be marked in an obvious way, or another place with a long line might look right, even if it isn't.
- You have to pick up the boots, put them on, get the skis (and maybe poles/helmet), put them on, and put anything extra in a locker. These may or may not be in logical places in comparison to each other, and they seem to all involve long lines, which may or may not be the right ones (kind of like the DMV).
- It all made sense at each place once I knew what was happening, but I learned to make extra time to figure it out. EVERYONE wants to get to the slopes ASAP, and I didn't blame them -- except for the employees, and I couldn't always find them.
8. There was a waiver to sign, and other paperwork. You have to leave time for all this before the lesson, plus time to clomp over in unwieldy snow boots.
9. It was so much fun!

I look forward to taking it up again, after COVID dies down. Not that skiing isn't safe, but I don't have vacation to use because of work duties.
I hope your family enjoys it. Bon voyage, and happy adventures!