You clearly do not know, how much it costs..
An example, little change from £100 if granny & gramps plus 2 want to do an activity in London ..& that is not adding in kids pick a toy each at the end or, ice cream - that alone for 4 - no change from a tenner.
Yes, of course one can take a picnic, drinks etc & do it cheaply, & yes - plenty of attractions for free, kids go free etc but, at the end of the day, if you are grand parents & only see your grand kids a couple of times a year, if you are lucky.. that is what it costs..!!
Times have moved on - I am old ..looking as an example.. Paddington Bear experience for a family,,