At that age, kids take their emotional cues from the adults (and at times other kids) around them. So all you have to do is be honest, truthful, and a bit upbeat and your child won't really care. When they start to care (say bedtime, or bath time, etc.) they tend to care more about the disrupted pattern than anything else. So make sure the grandparents know what the normal pattern and things to do are. For example if it were my son at that age, I would do something like:
Hey, good news, You're going to get to spend some more time with your Grandma. You will get to back cookies, and play games, and have a great time. Me, I'll be fine, I don't feel very good, so I have to focus on getting better. You will get to 'do a favorite thing here'. We will see you in a few days, make sure to 'do lots of other favorite things'.
The important part is to make sure that you and the other adults frame it as positively as you can. You may feel sad about it, but your child doesn't know to be sad about it yet. If they start to miss you or feel sad, then a quick video call should do the trick. Certianly you want to show that you care, but not by teaching that this is a sad thing and everyone should be dramatic and mopey about it.
Focus on the positives:
- I love you
- I will see you soon
- You will have fund with Grandparents
- You will get to do a lot of fun things
Stay away (as much as you can) from the negatives:
- I miss you (becomes I miss you too but I will see you soon)
- I wish you were here (becomes, I wish I was there having fun with you)
- I want to come home (becomes, soon you will be able to come home, but right now you can play with the pigs (or whatever unique fun thing your grandparents have))
The main idea being if you stay positive then so will your child. Also when they do get to come home, make it special somehow. From a celebration to movie night, to whatever you do. Make sure to show that your happy their home.