What happens to our pets when they pass away?
If there was any way to answer this question with 100% certainty, then who ever had the answer would be a rich person and the hero of all parents. It's a difficult question to answer, and our answer doesn't validate or invalidate yours by any means - if anything it's going to be a similar or contrasting thought that will enlighten each of them when compared.
By including into our name of the word "Heaven".... we are implying that we believe our pets have souls that cross over into a place free from worry or pain, where they can play, sleep, frolic or run with any of the others that have crossed over before. We do not have a religious affiliation that we associate heaven with, and feel this is a personal decision for each pet owner to make.
For years a very popular poem and accompanying theory have been very popular. That is the famous Rainbow Bridge. I have not found an author listed for this poem, so I won't list the text here, but you can definitely easily research it out and look up this beautiful poem. They're an excellent source for pet loss resources in addition to what you will find here.
Anyone who has ever looked deep into their pet's eyes know that there's a soul inside - this writer believes the same. There are religious aspects to believing in a soul for many and some have been taught to believe that animals don't have souls. But considering that we ourselves are just a very advanced form of animal, I would find that hard to accept. I've known the unequivocated and unconditional love that an animal can give back to you, and surely love that pure cannot be soul-less.
We believe you should believe what feels right in your heart and for your experiences and not be judged by others for that.