tokyo to osaka

i'm travelling with my family 2adults+2kids to tokyo/osaka. tky, staying at shinagawa area. should i take shinkansen to kix or take domestic flight? seems like it's cheaper to take flight.
at kix, i'll be visiting universal studios, osaka castle, aquarium and do a side trip to kyoto.
which area in osaka do you recommend me to stay for easy connection to the many places that i plan to visit?
i'd booked osaka granvia but was told that limousine bus doesn't go to hotel but to bus stop 3-5mins walk away - herbis osaka. i'm wondering if walk with many luggage is possible during rainy season in june?

I'm travelling with my family 2adults+2kids to tokyo/osaka. tky, staying at shinagawa area. should i take shinkansen to kix or take domestic flight?

It is personal preference. As for cost, it depends on what kind of fares you are able to get hold of.

which area in osaka do you recommend me to stay for easy connection to the many places that i plan to visit?

Central Namba or Umeda are most convenient.

i'd booked osaka granvia but was told that limousine bus doesn't go to hotel but to bus stop 3-5mins walk away - herbis osaka. i'm wondering if walk with many luggage is possible during rainy season in june?

If you stay at the Granvia, I recommend to take the train, instead. There are direct rapid trains between Kansai Airport and Osaka Station.