Path negotiable with baby carriage in rain

by Diane
(Woodinville, WA)

I am trying to figure out what materials to use for a safe path on rainy days, and is slightly sloped area in the shade. I live in a rural area outside Seattle, WA, where it rains about 10 months a year. The daughter of my neighbor on one side moved in on the other side of my home. She has twin babies, one of whom has needed many surgeries, so relies on the frequent help of her parents, so they often cross our yard with her double wide baby carriage. We are happy to have them walk through our yard, using an old farm gate that was hidden under a large evergreen tree. We have cut an opening in the branches, and tried to make it welcoming, but soon the rain will start, and the grass will constantly be wet. The path would be about 20 feet, sloping up perhaps 8-12 inches over that distance from the gate to a driveway. We don't know what materials would be non-slippery, not trip hazards, and easy to negotiate, considering the rain, shade, and slope, and affordable. Our driveway is black asphalt, and our house is tan with burgundy shutters.
Can you help? Thank you very much!