For 6 months you can still leave Malaysia and come back 1-2 days later. A quick trip to Singapore or Thailand would suffice. If it's longer than 6 months then immigration will start questioning so best to contact Wisma Putra in these cases

Be careful when traveling through airports/borders with children when their parents are not around though, make sure you have letters from parents allowing the granddaughter to travel with your mum, way too many human traficking and kidnapping issues out there.