No, it sounds fine. And ligthen up on your children. They would not put their baby in harms way.
You had your chance to raise your children, now let them do the same. Support, not questioning and being overbearing, makes for a good grandma.
Congrats on the grandbaby.
Bring a blanket with to tuck in and around her to make h er a little more secure in there.
If she has good upper body strength and you are comfortable with it, it should be fine.
I wouldn't recommend pushing her really high or anything. But she is plenty old to sit in the swing while...