I'm thinking about getting one of those DSTT cards that my friend has. I'm thinking of getting this one.
http://www.memorycardzoo.com/1918/DSTT/NDS...cro-SD/#Reviews
But...
1. How do I convince my parents that they're safe?
2. Are they even safe?
3. Does anyone have one?
4. General things like how to use it.

1) I couldn't say, i don't know if they are or not.
2) Probably not, if there was slot for expandable memory then why would it need an adapter card?
3) No.
4) I assume you'd plug it in somewhere on your DS (not DSi) and then you'd be able to plug more storage into that.

2. Depends on the rom. Some/most roms are safe, while some are made to brick your device.
4. Plug it into your computer with the provided cable, download roms onto it, plug it into your ds, open the roms and play.

http://www.dealextreme.com/details.dx/sku.11811

Just get this.

DSTTs are awesome! I got mine from Deal Extreme about a week ago . Its only about £4 from there.

1. How do I convince my parents that they're safe?
There's nothing unsafe about them

2. Are they even safe?
I think they are, I've had no problems with mine

3. Does anyone have one?
Yes, me

4. General things like how to use it.
Well, you get your MicroSD card and plug it in your computer. Format it to FAT(32) and then download the DSTT firmware from their website. Then you just drag the files onto the root of the MicroSD and once you have all the games you want on there (make sure you have the actual game otherwise its illegal ) just put the MicroSD into the DSTT. Put the DSTT into your Nintendo DS and switch it on. The DSTT will load up and it will have all your games/roms on there.

Hope that helped . If you need any more help, just PM me.

EDIT: Heres the DSTT I bought: http://www.dealextreme.com/details.dx/sku.15613 . Only $5 dollars and free worldwide shipping .