My husband enjoys metal detecting and has several detectors. I've found a link that reviews detectors and has a link to a metal detector's forum. The main page has some recommended detectors for beginners.
http://metaldetectorreviews.net/
Know that this is a hobby that can get obsessive. You must have permission to detect on private property. You should check your county's ordinances about detecting. Some rules of etiquette include always filling in your holes and leaving the area cleaner than you found it. We always take a trash bag with us since we end up finding lots of old soda/beer cans, nails and other garbage.
You can spend several thousand on a detector, but should be able to get an entry level for a couple of hundred. One of our detectors is a $50 one found at Big Lots a few years ago. It is very simple to use. You want a fairly simple detector for your daughter. If it is too complicated she will lose interest. You could also check area pawn shops since detectors can end up here used but at decent prices.
It won't make much sense right now, but the detectors have various sensitivity levels. What do you think she would like to search for--coins, jewelry, gold, etc. The detectors work by eliminating various ground minerals and focusing on a specific "target". And then you have the depth range. Obviously, the models that detect deeper can cost more. And you mentioned the beach. Some models can be used in the water, some can't.
Other things your daughter will need to detect: heavy gloves, digging trowel, treasure bag. It is a fun hobby--and I say this as someone who's only found soda cans.
Links to detectors:
http://www.garrett.com/hobby/hbby_division.htm
http://whiteselectronics.com/?gclid=CJb5hbfUtp8CFQUhnAodI1tx0g
http://www.tesoro.com/
http://www.minelab.com/
for idea of pricing:
http://www.kellycodetectors.com/detectors.htm