Have you considered something like the Tern GSD or a cargo bike with seating?
https://www.ternbicycles.com/us/bikes/471/gsd
http://dutchcargobike.com.au/shop/bakfiets-riksja-elderly-people/
http://dutchcargobike.com.au/product-category/family/
The biggest size in the Marathon Plus is 47-622 (700 x 45C)
Check your rim width to see if the 47-622 (700 x 45C) will fit
https://www.schwalbe.com/en/reifenmasse.html
Nice looking bike and just an observation, but I think you need to get your priorities right.
It looks a bit crowded in there and the new toy should be taking pride of place