I too am just starting up a junior choir (having left the school where I built up a highly successful one over many years).
I can't recommend the Sing for Pleasure Junior Song Books highly enough. They are A5 size with only a few pages in each and you can buy the whole set for under £30, I think (check the SfP website). They are full of echo songs, action songs, simple rounds and part-songs. I particularly like 'The Forth Bridge' (I pinched some excellent actions to go with this one from a choir I saw perform it at a festival once

) and when my choir did 'A Young Austrian' at a big concert several years ago they almost brought the house down!
I would reiterate the recommendations of Voiceworks, although I have to admit to being disappointed with Junior Voiceworks.
Generally, anything by Lin Marsh and Jan Holdstock is very good. Lin's 'Spooky Songs' have gone down a storm in the past. Jan's 'Tell Out the News' is an excellent Christmas piece which builds up three separate verses which can all be sung together at the same time.
Alan Simmons also produces a lot of material in attractive collections. The quality is a bit variable but he is very good at writing partner songs (where the children learn two or three separate melodies which then fit together). A lot of his songs are humorous and bouncy (I particularly liked 'Winter Sport') but I actually like his slower songs better. Two real winners, for me, are 'Butterfly' and 'Creatures of the Deep' although with all his songs I tend to play around with them a bit and usually get the children to sing the two parts separately before putting them together.
Alfred Publishing do masses of good stuff - Level 1 is excellent for beginning choirs. There is lots of slushy schmaltzy stuff (hooray!

) which is especially good for Christmas (Level 2 in particular has quite a lot of quodlibet-type stuff). The composers I've particularly enjoyed are Sally K Albrecht, Jay Althouse and Don Besig/Nancy Price. They also publish a book called 'Grab a Partner' which has easy two-part arrangements of songs such as 'Kum Ba Yah' and 'Shoo Fly'. We sang two of their particularly slushy ones at the weddings of two teachers and there wasn't a dry eye in the house! One of them - 'Go Forth with a Song' I've also used successfully (ie hankies out again

) for Year 6 Leavers' Assemblies. PM me if you would like to know exactly which Alfred pieces I've done.
Youngchoirs.net publishes a series called 'Junior Choral Club' by Jo McNally. At first I thought these looked really good but I'm afraid I've never managed to be inspired enough by any of them to do them successfully. The piano parts are meant to be simple but they are also pretty horrible in places. A real shame...but don't dismiss them, do have a look - I heard one choir perform 'Engine, Engine' quite effectively at a festival.
Doreen Rao's 'Choral Music Experience' has some good things - look at the junior anthologies. 'A Manx Lullaby' is unison with piano and very beautiful (veers between major and Mixolydian) - especially if you can find a good flautist from somewhere.
I would reiterate the cantata recommendations given so far. 'Jonah' is not THAT easy but great when you get to that stage.
John Bryan has written some lovely cantatas - my favourite being 'The Selfish Giant'.
Oh - and I have to also put in a plea for you to try unaccompanied singing. If the children always, always sing with a piano they will use it as a crutch and will find it increasingly difficult to sing without it. They will also not take such care with tuning. Solfa is a tremendous tool for improving intonation and general aural awareness - PM me if you would like to know more. Also if you are always behind the piano you are 'distanced' from the children - unaccompanied work allows you much more eye-contact and 'connection' between conductor and singers.
Hmmm - I think that's probably enough to be going on with for now!
Best of luck and enjoy!