I haven't heard of that brand. (Psst... You might want to get a new piano in case this current one turns into a money pit.)
The piano is out of tune. The pegs inside unwind and the pitch lowers, so a C# key sounding something around a C natural makes sense.
Twenty years without a tuning is a looooooong time. The piano is probably nice and comfy with whatever position things have settled into -- And after you have it tuned, it will want to resettle back into its current position. I'd be very confident in betting that it will take at least two tunings to get it to hold pitch.
Assuming it can hold a pitch.
? You said the piano hadn't been tuned in about 20 years... They can out of tune within hours or a few days, probably in a few months something will be a little off. Whoever tuned it didn't tune it a whole tone lower. It's just settled out of tune. Actually it could slowly be going more and more flat, although I would imagine it would be moving veeeery slowly after 20 years without a tuning.
Temperature and humidity changes will affect the tuning. That changes the pull on the strings, the ability of the pegs to stay in place on the board they fit into, etc. It's like weathering -- slowly over time, the environment is going to force it out of alignment.
Inexpensive keyboards are also not always perfectly on pitch. I've got one that's a quarter flat. It's a cheap keyboard though and does what I want it for -- ear training and portability.
If you're really serious about it, maybe have a piano technician take a look at it (except that costs money), and have that person look at it before spending money on tuning. If it's not worth it, trash the piano (you might not be able to even give it away if it's not great) and invest in a new/slightly used piano. Or have someone tune it (a couple times at first) and see how it goes. If you have to keep tuning it though, that will add up over the amount of time the instrument is used, not to mention having to put up with a lesser instrument than some -- It affects your playing. A lesser instrument will limit what the student can learn. -- But if you get a decent new/used piano, then you'll probably spend less money tuning it and will have a better experience with the instrument in the long run.
And if I had to bet.... You'll keep this piano. You'll probably get it tuned, but it will take at least two tunings. It might even sound worse after the first tuning -- Some keys will slip back faster than others. It's a family heirloom. It sounds like you just want your kids to have a taste of music like you did (not to be insulting). The piano tuner probably isn't going to tell you if the instrument isn't that great. (Although for some reason the people I've met who tune pianos don't seem to like tuning them at all.) The piano teacher probably isn't going to complain too much about the instrument -- They'd probably say something similar to what I'm saying or what others here would say. Maybe if your kids really make progress you could invest in a step-up instrument, a better upright or possibly even a grand.
Is this it out of curiosity?
https://www.users.on.net/~mec/langholm/eddyola.jpgAre you in Australia?