HeartGold/SoulSilver questions

Hi, folks. I have never played any Pokemon games before. Our twin boys have their sixth birthday coming up. They already each own a DS, but they've never played a Pokemon game. They've asked, but I've held off until I felt they were able to read well enough to play on their own. Well, the time has come.

I bought one copy of HeartGold and one copy of SoulSilver, thinking I'd give them each a different game. But then some questions popped into my head. Here you go:

1. Is the infrared capability referred to in conjunction with the Pokewalker already built in to the DS? Or will I have to buy a separate peripheral that facilitates the infrared data transmission to the Pokewalker? Is there another way to transfer data to/from the Pokewalker other than infrared? If so, is it free.

2. What peripherals/add-ons are needed to facilitate DS-vs-DS gameplay? Is there an actual physical cord that connects the two? Or is it a wireless device? Or is this built in to all DS's?

3. Can the boys play versus one another even if one has HeartGold and the other has SoulSilver? Or do they each need to have the same version of the game?

Thanks in advance.