There are a few preserves that say they are "safe for animals once dry". I bought some from Wilkos, I think it was decking stuff but they also had some other "pet safe" preserves but it only can in 5L tubs which was just a silly amount!!
I know I got really stressed when looking for an animal safe wood preserve as so many exterior stuff didn't mention whether it was animal safe. I'd stick to one that specifically says "safe for animals" on the tin.
Hope I haven't waffled on too much!