Trying to find a live action movie with CGI monsters

Sounds like The Spiderwick Chronicles.

From the Wiki description, we see some of the main points:

Single-mother family, new house, goblins:

Recently divorced Helen Grace (Mary-Louise Parker) moves into the Spiderwick Estate in the woods with her children when it is given to her by her elderly aunt Lucinda (Joan Plowright), though identical twins Jared and Simon (Freddie Highmore) and their older sister Mallory (Sarah Bolger) do not want to move from New York. When Jared uncovers a dumbwaiter system behind a wall, he finds a monogrammed key and discovers the study of the late owner of the estate, Arthur Spiderwick (David Strathairn). Jared then uses the key to open a chest. In it, he finds Spiderwick's field guide to fairies; although an attached note warns him not to read it, he does so anyway. When Jared explained about his discovery on the existence of magical creatures, Helen and Mallory don't believe him.

Device to see the creatures that are normally invisible, house protected by magic:

Soon after, Jared meets a brownie named Thimbletack (Martin Short), who explains that magical creatures are normally invisible, but can reveal themselves at will. He tells Jared about a protective circle that Arthur Spiderwick placed around the house and gives him a stone with a hole through which he can see fairies.

Monsters want the book:

However, a shapeshifting ogre named Mulgarath (Nick Nolte) wants the field guide for himself so he can rule over all fairy-kind. He sends his goblins, led by Redcap (Ron Perlman) the Pompous Goblin General, to obtain it and they kidnap Simon, mistaking him for Jared. Meanwhile, Jared meets Hogsqueal (Seth Rogen), a hobgoblin and bitter enemy of Mulgarath because Mulgarath killed his entire family. Hogsqueal gives Jared the ability to see fairies without the aid of the stone by spitting in his eyes.

"At the end they travel to some place where they meet someone they thought was dead (grand father possibly). They want to take him back but he can't go because if he returns he would age and die.":

Suddenly, the Sylphs appear, bringing Arthur, as he was allowed to visit his home since the book was now safe. He cannot remain outside of the fairie realm, or he will turn to dust.

Here's a trailer to see for yourself if it sparks memories: