Great question. First let me admit I work for a major retailer of school music products. With trumpets, like any other consumer product you get what you pay for, and if a deal is too good to be true it probably is. There are many brands of trumpets that have been around for 20, 50 even 100 years. Buying one of those brands is a safe bet because the companies producing those products have a lot to loose if the quality is poor. Check the brand's web site for clues as to how long they have been around.
Another safe bet is to buy from a music store that has a repair shop with a good reputation. Chances are if they must service what they sell you will get a quality product that will serve your child well.
Finally, a local music teacher may be a good source of information about the experiences of other parents and students.