I had good luck with these https://www.amazon.com/Nylabone-Puppy-Chew-Teething-Rings/dp/B002C7GQ0C (they don't last long and I wouldn't let them have them unattended) but in the end of the day, a cotton cloth, tied in a very tight knot, wet and put in the freezer as an attended toy helped the best. =)