The main question would be, are you happy with the internet speed you get at the moment?
And as long as you and the grandchildren don't want to all watch videos online at the same time the speed required for 4 people just the browse the internet is actually very low. I've had over 100 people using a 4Mb/s connection and it was perfectly usable for normal browsing (OK, only 2 or 3 people could watch online videos at once but people shouldn't be watching things like Iplayer at work so that wasn't an issue)