How do I increase my dog-walking price?

Hi, I am a high school student who has been walking dogs since I was around 9-10 years old.

When I first started walking (around 5 years ago) I charged $5 per 30 min. walk. At the time it seemed like a reasonable price because I had no experience and was in middle school. Now I have done research and decided that I should increase my price. My biggest fear when doing this is losing clients because I only have 3.

I have been walking the same dogs for 5 years at $5/30 min. and I'm not sure how the owners will react to an increase up to $8/30 min. I know both of these prices are low but I walk multiple dogs for each family so it will be around $16/30 min. for two dogs at the same time. So, how do I go about changing my price without losing customers?

I also wanted to know if people charge extra for preparing dogs for the walks. It takes me about 5-10 minutes at the beginning and end of each walk to put boots and coats on all of the dogs. Should I be charging extra for this?