For anyone willing to stay with a child at Hotel des Arts Saigon, please consider this info from the property who just answered me:

"Based on your concerns, please be informed that our hotel rooms (30 square meters) can accommodate maximum 02 adults and 01 children under 09 years old, sharing the bed and having breakfast with parents. In this case, we suggest your daughter could share the bed with parents. Otherwise, kindly refer to our highest room catergory Executive Studio Suite room (60 square meters) that can be arranged 01 extra bed with its surcharge per night."