Looking for very easy online computer chess game for children just learning.

Look into Majestic chess. The problem is that your children might get hooked on it; it's kind of like playing a video game. But the good part is that we were able to make our son earn his majestic chess screen time by doing a bunch of other stuff : )
It's only good for taking players up to 1100-ish. It puts your in a adventure where you have to apply what you've just learned (starting with how each piece moves, then on to pins/skewers/forks, development, etc.) to fight battles to assemble your army (which is rooks, bishops, pawns, etc.) to finally fight the boss of each level. If you are good, you don't even need to assemble a full army and just go after the boss (which my son and I was now able to do, and can finish the entire game in a day! It used to take us days/weeks just to pass a level.

maybe something like this?:
http://shop.chessbase.com/en/products/fritz_and_chesster_part_1_version_3

Of the apps I have tried, Chess Pro (by Christophe Theron) for iPhone and iPad is my favorite for both kids and beginner adults. At the easiest level, the computer moves the pieces completely at random, so even the newest of new beginners can learn from it.
But what I really like is that the interface for analyzing games is visual and extremely intuitive. You don't have to keep pushing a "help" button for every move or learn chess notation. It shows the best move(s) through green lines. And it also shows threats with red lines. This is helpful for understanding why some moves are good and some are bad. Very few other apps/programs do this.

On a PC, the free Lucas Chess (http://www-lucaschess.rhcloud.com/index.html) has "opponents for young children" named Monkey, Donkey, Bull, etc. that play weakly - lower 100's perhaps.
Example games are here
https://www.youtube.com/watch?v=zmMLYXSa8Ic
https://www.youtube.com/watch?v=77ZWI8I7hfE
You can also choose from lots of engines with ELO Ratings from 269 up to 3400.
It's my favorite chess program - and I also have the pay-for-it Chess King.