Love is easy. Just tell them. I suspect they would be thrilled to hear it and i suspect that you would be upset if something happened to them and you hadn't told them. It really isn't hard. Just say "mom (or dad) i love you" or "you know i love you. I cannot tell you how thrilled I am when my children (all grown now) tell me they love me. It makes me feel special and makes it all worth it.
Say it in your way. Every child is different and you need to find your own way to express your love. Some tell, some draw, some sing, some play music, some kids just get goofy. Do it your way, your parents will love whatever you decide.