Advisor to spend some funds to buy things that make group members feel better working at home. Is it selfish to request a tablet and stuff?
Surely these are precisely the sorts of things that your advisor envisaged buying when they offered to get "items to make you more productive or happy working at home"? A decent monitor (and keyboard, and mouse) for your laptop are fairly essential if you're going to be using it all day every day for the next N months. A tablet is perhaps a bit more of a luxury, but hardly excessive. Remember, as a fraction of the total cost of supporting a PhD student, an iPad is small change.
If you're concerned, phrase your email making it clear that you are presenting a list of suggestions rather than demands, as you're not sure how much money the advisor will want to spend.
Would it be selfish for me to ask my advisor to fund me to get a monitor and tablet, those things?
Not at all! It is never selfish to ask, though it might be selfish to insist, and it might likewise be selfish if you did not make it clear that you understand your advisor has the final say in this.
It is an odd feeling asking your advisor to pay for things for you, isn't it? Many students are afraid of coming across as entitled, but the better approach is to be matter-of-fact and professional about it. Simply explain (in a short message to your advisor)
The item(s) you want to get;
In a nutshell (1 sentence), why this item would help you be more productive and comfortable working at home;
The specific estimate on price or price range.
Don't phrase it as a demand; just state what you are thinking of, and then ask: "Would it be possible to reimburse this item?"
My advisor proposed to get us some items to make us more productive or happy when working at home.
Based on your this, I agree with the top answer that it is overwhelmingly likely your advisor will be fine with getting you these items. But articulating (1), (2), and (3) above will help make this more concrete, and phrasing it as a question makes it easy for your advisor to say no if they think that one of the items is excessive.
It probably depends on the funding source or grant and the cost of the item.
Beware that in principle stuff purchased through a grant does not belong to an individual but to the grant organization or to the Uni administering the grant. (Our work computers have a Uni sticker on them and are part of the official institutional inventory.) Thus, you may have to return said item once the situation is closer to normal.