I like the art and the setting for the story but its way too stupid regarding the grandfather's wish ...
How did he become a grand-FATHER in the first place?
In chapter 2 the girl said that they are his adopted grandchildren; as in his son adopted children and they became his grandchildren. At least that what I got from her explaining it would be like having their father at school with them.
There could be 2 ways this setting could have worked out much better:
1. He could of been in a bitter marriage that he was roped into and never got to experience a proper romantic relationship with someone and he wished he could of done it.
2. After getting older and getting nowhere with women, he adopted a son to give himself purpose and that son grew up to have kids of his own. (this way the story would fit as him being a grandfather but never having a girl and it would still work for the grandchild who is in love with him since they would still be not blood related)