It's a huge responsibility for your MIL. Is she an experienced traveller? If it was an escorted tour maybe. A 7 year old has a lot of energy and curiousity, as you know.
We took our kids over to the US for 3 months when they were 7 and 9 years. They both remember all of it (I made them to journals as they were missing school. I think those FAMILY holidays are legendary and if I were you, I would wait until you can all go away together. May not be India, maybe New Zealand, Vanuatu or Samoa (relatively cheap, kids eat free etc) or even Bali as you live in WA, but it's YOUR family's memories.
