Today my 6 years old nephew and i played ManicDigger . He enjoyed the Minecart feature , but as i see the game is a little poor compared to Minecraft .

So i decided to purchase Minecraft .

What essential mods ( for example LB Photo Realism ) do i have to install if i want to play the game in peaceful mode ?

Go to your options. Go to difficulty. Set to peaceful. Simple! You don't need mods for that.

If you'd like a bit more to do that doesn't involve fighting, I'd suggest checking out "Better than Wolves" and "Industrialcraft." I don't think they work together, but each one greatly expands the tech tree and adds tools and items to play around with that don't require fighting.

I'd also consider Mo' Creatures. Mo' Creatures adds a great many animals and monsters - some of which obviously will not spawn in peaceful mode - such as tameable horses and adorable little birds that fly about and...uh...well, last time I used it, that's about it. I can't say whether this is still compatible, but I do know I used to use Industrialcraft and Mo' Creatures together.