i mean that when i'm playing other ds games on 3ds it normally goes into sleep mode but when i'm playing animal crossing wild world my 3ds won't go into sleep mode at all

Since Wild World is a ds game, im pretty sure it is fine. I think that happens cuz the 3ds goes into ds mode and the ds did not have sleep mode.

i don't think so, i'm pretty sure every DS game should let you enter sleep mode...