Try adding an electric lamp timer to the mix. Fire up their bed about
an hour before you turn in. Place them on it once it's nice and
toasty.

I used mine in the opposite way: have a geriatric kitty, want her to
sleep with me. I set the timer to keep her bed warm during the day
(cycling on and off so it wouldn't get too hot) and then shutting down
for the night about the time I'd turn on the e-blankie on my bed.

Yep, it's terribly sweet when they get old and boney enough to crawl
under the covers.

HTH

--Karen D.