I'm with the others. When it comes to tech related gifts (phones, computers, cameras, etc), it can be rather hard to give something adequate unless they have mentioned something specific they want. Limiting yourself to around $50 each is also going to restrict yourself to mostly bottom of the barrel throw away phones unless you get lucky and find a used one that fits the bill.
Have you discussed this gift idea with their parents? For one thing, they may have strict limits on screen time, and not be appreciative of phones as gifts. On the other hand, they may also have more insight on what would be ideal (but there's still the problem of the costs). Due to how individual these devices can be, you may want to just give them something like a home made "gift card" good for a phone with your $50 limit. Obviously they couldn't take that to the store, but it gives them something to open and they get to be a part of the process to get something they would like. Depending on their age, they may not care but my wife and I have done such things with each other in the past with tech gifts.
I would suggest revisiting the tablet idea, as well. Part of the cost for a phone is the hardware and programming needed to make use of such functions. If you buy a small tablet you'll get a little more bang for your buck by only paying for features that would be needed. They are likely going to be about as durable as a larger phone, and you may be able to get protective cases for them as well.
When it comes to Wi-Fi only use, there's not much to worry there because everything is more or less universally compatible. What is more important is processor, storage, memory/RAM, OS version, screen resolution, etc. Those are the specs that can determine how well the device will run.