I had a 3 level posterior/anterior lumbar fusion almost 3 months ago. I might be able to lift 20 lbs now. There is no way I could have done it right after surgery. They generally say you cannot lift anything heavier than a coffee cup until you are 3 weeks post op. Then I believe it's a 10 pound limit until 3 months. Everyone heals different, and I'm not sure what part of your spine is being fused -- that could also impact how much you can lift.
The mantra after surgery is "no bending, lifting or twisting".
You don't want to risk reinjury or not fusing. I know you love your grandaughter and are worried about her. Perhaps you could find someone to come in and help you take care of her until you're strong enough to lift her.
There are plenty of "nannies" out there that need work, and they would be under your supervision in your home. They might even be able to help you at the same time. That's the best idea I can come up with.
btw, who's taking care of you when you get home from the hospital? If you don't have a spouse or friend helping out, read the tips at the top of the page, they're very useful.
My surgery went very well, I'm doing great now, I went back to work 3 weeks ago, and now I'm going to the gym again. A positive attitude towards recovery is imperative, so is walking. Short walks 3 or 4 times a day.
I wish you a very successful surgery.