When my son was finally old enough and mature enough to have a phone, I put him under my plan on AT&T with serious restrictions. I have unlimited text messaging which is great. Before I got him his phone, his uncle got him a firefly before he was ready for it and then upgraded to another phone. My lovely son downloaded "free games" which were only free for a certain time period then they had to be paid for. He also managed to get onto the internet but didn't have an internet plan which again was more money shell out. Within 3 months his uncle had to take the phone from him because of the three months of bills over $300.00.
Long story short, gift your kids the phone of your choice. Reward their disciplined use of the phone with less restrictions and put more restrictions on the phone if they are out of control until they become 16. At 16 it is a whole new ball game, they must pay for half of their phone costs. This teaches responsibility and you get to see how mature they are or are not.
Any phone will do but remember internet access costs more and on AT&T you will not be able to restrict incoming calls just outgoing. I hope this helps.
Yes they offer a deal on the iPhone but the internet access will cost you $30.00 plus the $9.99 a month for each additional line and you will be hard pressed to restrict their internet access. Not the choice I would make for my kids and their first phone. FYI two of my sons have had their phones stolen and they break too so get the insurance for the phone. It is helpful since any of the phones you will get will run you more than the $50.00 replacement costs through the insurance.