Paperback about siblings running away

[thehunter]

This was a YA book, probably written in the early to mid-1980s. The cover had a dark blue border and a picture of two kids looking frightened and hiding behind a bush, crouched down low. The kids were wearing backpacks and had identical or very similar haircuts, something between a bowl cut and that "curtains" style that was popular in the '90s. I believe one of them was wearing a purple and green striped rugby shirt. A dark figure in the background seemed to be looking for the kids.

I remember very little about the plot. The children were brother and sister (the sister was a little older) and had run away from home together, perhaps because their parents were getting divorced and they didn't want to be split up. I remember them packing supplies in their backpacks, and I think they were planning to live in a public park for a while. The girl may have cut her hair like her brother's, in hopes that anyone looking for them would mistake her for a boy.

If anyone has any idea what book this is, please let me know! Thanks!

Edit: FOUND! The book I was looking for is At the Sound of the Beep, by Marilyn Sachs. The kids are hiding behind a tree, not in a bush, and I don't know where I got that idea about the rugby shirt, but this is definitely the right book. Thanks for your help!