Hello. I'm from Romania and I've been utterly obsessed with the Reksio games ever since I was about 7. (I am almost 17 now) Does anyone speak English here?
The reason I joined is because I always thought about the games, and tried to find explanations ( even though it's just a game and a lot of things make no sense xD )

But I just recently realized something. It makes no sense for Kretes' grandchildren to be the cause of his travel in time. Here is why:
Kretes talks to his grandchildren about how the time machine was built > Kretes falls asleep, then we hear the voices of his grandchildren talking about the machine 'hmm a huge blue sofa?'. That can only mean that the grandchildren do not know -why- their grandfather went back in time.
We can also see that it is nighttime, and at the end of the game, it is daytime, and the grandchildren have the Nostradamus robe. They say they are tired and are going to go to sleep.

Throughout the game the grandchildren's mischiefs in the past result in a future where Reksio is taken away and Kretes never meets Molly again. 'If you don't want this to happen you have to go back in time and meet me'. From the grandchildren's perspective, they left at night and returned the next day. So... it's a paradox for Kretes to have gone back in time when he was younger , tell his grandchildren about it, and the grandchildren to become the initial reason of his time travel.
In Reksio and the Magicians at the end we see the golden casket that Reksio put his paw on in the Ancient Egypt. It could've only gotten there (Magix) because of the grandchildren. (I read an article where a game producer said that the grandchildren went back in time for fun and they got the casket as a souvenir, but their time machine also broke so they put the letter in the casket and told a magician to place it in the Magical book. )

Also, Reksio and Kretes know how to make the time machine from Nostradamus' letter, but Nostradamus (grandchildren) knows from Kretes.
I think that without the introduction of the game, when we see the grandchildren finding out about the time machine from Kretes, the rest of the game makes sense. It should be like this: One day when technology is more advanced, and Kretes is an old mole, his grandchildren make a time machine. They alter the past so they have to call Reksio and Kretes back in time to fix what they've done. Don't you think?

Yeah, most of us can speak English.

This is only one of many Time Paradoxes in the series. I once tried to explain it... thre is one thing not correct in your explanation: time machines still weren't invented, when Kretes' grandchildren traveled to the past first time... unless they were invented some time later.
Did You play "Reksio and captain Nemo"? If so, it will be easier to me to explain...
As You know, in "Capt. Nemo" there is another time machine, invented by Kretes' ancestors. However it sank along with whole city, but many years later it was been found by Nemo. Nemo found it thanks to the help of Reksio and Kretes, but... maybe in future someone else found it with the help of Kretes' grandchildren. Then they traveled to the past and... [story from "Reksio and Time Machine"].

What do You think about it?

Oh I'm so happy Yes, I played all the games a couple of times. What do you mean, they weren't invented? Hmm, weren't they invented in the future where the grandchildren came from? By invent, I mean someone coming up with how to make it work..
And yes, but the ancient time machine (Mu) works differently... But there's something really odd though. The mole 'tunnels' have been there since the Dinosaur Age. How come they just coincidentally also help you travel back in time/to the future if you happen to have a working time machine?

Hmm, remember when Reksio and Kretes visited the Dinosaur age where they met the so called 'ancient moles' ? I think they created the ancient city that sunk. And the time machine (Mu) as well... I just realized that it's probably a parody of the Atlantis ^^

No, it's a parody of Mu.
http://en.wikipedia.org/wiki/Mu_(lost_continent)

Oh I see. Never heard of Mu as a lost continent ^^ My bad!