For most parents renting an instrument is the best option. It is easy to exchange the instrument when the child outgrows it and usually a repair plan is included. Some parents prefer to buy an instrument. Buying is fine as long as you know what you are getting. Sometimes a used instrument that has survived a year or two in the hands of a child is going to be better than a "new" instrument from eBay. There are companies making incredibly cheap instruments that are not playable. If you find a used instrument and aren't sure, send a link to me and I will give you my thoughts. Below is a list of suppliers of instruments for renting or purchasing that I have found to be viable options. Read the fine print when signing a rental contract, most companies require you to rent in the summer to get credit toward a purchase.
Your child's orchestra teacher will measure them to make sure you purchase the correct size instrument.