I think it all depends on the group you currently have too. I'm newer to having my home daycare (my 3 year anniversary is coming up soon!) but definitely not new to childcare. My first group, I would have done this with them for sure had I thought about it lol. Everyday they were excellent so doing something special once a month would have been great. I have since had a few parents who I wouldn't necessarily want to do something special and above and beyond for monthly lol. I do regularly tell them when I appreciate something they have done or said though. Like you said, it's a word of mouth business and my past clients have sent so many my way that I still tell a couple of them when I have openings so they can spread the word lol

If it works for you and you enjoy doing it, than keep on doing it When/If it becomes something you no longer want to do than stop. It's all up to you! I bet all the parents loved it!