Thanks again all
Actually she has no complain with any rented regulator with smaller mouth piece, so far. But, our local DS has new instructor who very concern with safety. Now, they refuse her for bubble maker. They said their equipment is not suitable for kid as all are designed for adult. Then, they advised us to purchase gear special for kid. I understood there's special BC for kid but I have no idea with special regulator for kids. Further they clarify as any light and very compact regulator with small mouth piece. Yes, they give me example of Proton Metal She dive available for sale (not for rent) at the shop but the price is very very expensive (more than USD800 bucks here). So, I am looking for other less expensive alternative.
Ofcourse I am very concern with the safety - that's why I am also asking wheather the big price difference between mares she dive and rebel or other regulator is really relating to safety.