Can anyone help with things to do in Athens for 3days with a 12 and 4 year old?

What time of year are you going? Are you on a budget?

Assuming summertime nice weather, one thing I might suggest is the funicular ride up the Lycebettus Hill, maybe around sunset.

Otherwise I'd be looking to take a boat. Go to Pireaus. Take a boat ride.

Of the ruins, my guess is that maybe the Temple of the Olympian Zeus might impress, and then it's a short hop over to the Roman Stadium, where they might enjoy running around like Olympians, and resting up in the National Gardens, where I enjoyed the turtles.

And I've yet to meet the small child who didn't like lighting candles in churches. Byzantine churches are full of them.

Α few sites that might help you pick out things to interest them & you.
http://www.nylou.com/html/ent/712/en....list.4.1.asp?
http://www.breathtakingathens.gr/
The 4 year old might enjoy the 'happy train', takes you on a ride around Plaka/Monastiraki!
http://gogreece.about.com/od/athenss...happytrain.htm
The soldiers (Evzones) at Syntagma Square,
http://www.athensinpictures.com/evzo...ns-greece.html
I also recommend the trip up Lycabettus Hill ready for the sunset, but any time of day is good for the all round view of Athens.
There are so many historical sites in close proximity & walking distance of the centre, but only you can decide which are within the interests & capabilities of the children.
Take a look at the choices & ask about specific ones if need be, it's no problem to give you more detail!

You will probably all enjoy the changing of the guard at the parliament - it is quite a spectacle. You can take your photograph with a guard, but watch to see where people stand. If you stand on the wrong side of the guards they slam their rifle buts on the ground and frighten the lift out of everyone who is nearby!!

I'm assuming you will go the the Parthenon?

You can also go to the National Archaeological Museum where the kids might be bored with the artifacts collections, but the statue collection is fabulous! There is a cafe there which faces into a nice courtyard.

Zeppole has a good suggestion to go the Temple of the Olympian Zeus - it is such a vast space and your imagination can run wild trying to imagine how this site would have been when all of the columns were standing. We also enjoyed the gardens for a little oasis in the middle of a VERY busy city.

I just finished an Athens photo album which will give you an idea of what you will see in the Plaka area of Athens.
Perhaps the kids will see something they would like to visit.

http://travel.webshots.com/album/570719132pdjjHG

Thanks folks! All the suggestions are very helpful.Planning on going in July. Are all the attractions close by to one another? What is the best place to stay for about $150 a night? What is the best mode of transportation?

Train ride is definitly on.

Tugozi -- it helps people to advise you if you will translate your $$ into Euro and put your maximum in Euro ... otherwise We have to do it, because most hotels quote in Euros -- here is a currency conversion site, for your future reference http://www.xe.com/ucc/convert.cgi

As you will see, $150 is only €113 -- and if you are hoping to get a "Quad" room for 2 adults and 2 children, in the Plaka district (where all the sights are) and in high season, that is not enough, Alas. Not many hotels even have quad rooms. I do think u need to think PLaka area, because in the July heat (it is the hottest month in Athens) your 4 year old will wilt, and you'll need to get back to the a/c and a rest without a long trek or a taxi ride.

The Trip advisor Greece Forum has run quite a few "threads" on the topic of family rooms. I suggest you go to that forum as a guest, and in the "searcH' slot put Athens Hotel Family rooms or Quad rooms, and see what comes up.