Not 'get well' please don't. Please.
Why not 'sorry you are poorly'? It's not a get well card or nothing, there's loads of things you could say that would be appropriate.
I know you mean well but by asking them to make a special understanding for your child you are putting your child's desire to send a card above these peoples tragedy and grief. I know it's culturally accepted for most situations to be child focused and kind of sweet when they don't quite get it right, but this is not one of those situations.
The man is dying. The woman is losing her life partner and closet person to her ever.
Please don't put them in the position of having to be polite about you telling a dying man to get better soon. It's not respectful of their situation, and you are asking them to swallow their pain to make it about the child and not about supporting them through the terrible terrible thing they are going through.
It's just not the right time to make people have to slap on a nice understanding face and to forget about their own pain & the inappropriateness of what you've given them.
I feel strongly about this as I know how alienating and upsetting it is when people do stuff like this.
People did this to me and my parents when my sister died. And again when my dad died. There was a lot of 'ahh they meant well' or 'isn't that so cute look at my adorable child' type of thing when actually, we were falling apart and to be honest, it's other people's jobs to think about our feelings at that time, not the other way round. If you can't do that, I'd recommend not doing anything at all.
Ok, maybe they'll feel different, but I don't think people should knowingly put that on those in terribly sad Abd upsetting situations. It's not fair.
I know you don't mean to just like all those other people didn't mean to upset and alienate me, but I do wish people would respond to death as something that requires its own social conventions, and not ry and force people to pretend it's not a big deal.