Both Liberty and Legend of the Seas are American ships so the service and facilities onboard are very similair.
I think your grandchildren will be well look aftered on both of them.
If you also want to consider pricing than Carnival are usually cheaper than RCI in my experience.
Carnival have a good deal and the ship is pretty new with lots of varying facilities that you will all enjoy however even though royal caribbean is good the ship is older and smaller than the liberty.Also Liberty is sailing from dover so no flights involve.
Both very different. Baltics is port intensive cruise - full of history and culture. Are the grandchildren intersted in that sort of thing? And Legend is doing a 'sunnier' itinerary. Personally I would not do either and would choose a large RCI ship that has all the facilities - like Independence out of Southampton next summer. Ice skating etc to keep them occupied. Legend is a lovely ship but older with not the range of activities or eating venues.