Mix the pumpkin that your Vet prescribed into their food.

Make sure it's just pumpkin, not pumpkin pie mix or with sugar or other ingredients.

Pumpkin with a bit of boiled chicken breast might work.

acadiafun1: Good idea... they didn't take to the pumpkin readily at first .... the bit of chopped chicken breast in it did the trick!! They, like most of us 'seniors' - our digestive system slows down so... out comes the prune juice & .... now the pumpkin!

My favorite over the years for a dog that needed some fiber was to add All Bran cereal to their kibble.

You said your dogs "occasionally get constipated." I would consider a kibble with a higher fiber content.