The Vanquish and Simplex are outstanding detectors.
The current Simplex bundle deal with the free 10X5" elliptical coil is a really good value. The only reservations I would have with the Simplex for a normal 12 year old are the weight (very sturdy detector that feels like it) and the somewhat blended audio in the 2 and 3 tone modes which can create a bit of confusion in the do I dig or not choice. Otherwise, if your child can handle the weight, the Simplex is an amazing, feature rich, keep one occupied for a long time detector.
For the Vanquish models, I would go with the Vanquish 440. It has everything you need in an easy to operate detector that has excellent balance with the 10X6" coil, is very adjustable, and most importantly has Minelab's outstanding Multi IQ simultaneous multi frequency technology which has very accurate target IDs (once you get to know them for your targets) all the way down to the edge of its detecting ability which depending on soil mineralization is easily 10 to 12" That ability in an entry level detector is unheard of and makes all of the Vanquish detectors, especially the 440, way more than entry level.
Get a quality handheld pinpointer by Garrett, First Texas (Fisher/Teknetics), Nokta Makro or XP. It will cost you $100 but it will also save you a lot of frustration recovering targets.