1. Your starting point is the choice or supply of materials

The children could be involved in collecting bits and pieces from home: in which case the sky is the limit. However, if you would prefer to restrict the scale of your enterprise you may wish to suggest smaller items: buttons, beads, coins, decorative papers, fabrics, ribbons, string, tickets, newspaper clippings, supermarket fruit bags, bottle caps.
Alternatively, if you know the effect you are wanting, or if you are hoping to teach a particular skill (weaving, mosaic making, a painting technique) you may prefer to provide or restrict the materials.

2. Where is the finished product to go?

(i) Consider the scale.
(ii) Should it be flat or more three dimensional?
(iii) What support does it need: a noticeboard, a wall, a window for a light supply, a point to suspend it from above.
(iv) Will it need supporting dowels or a frame and what should the base material be: fabric, mesh, polythene, card, paper, canvas?

3. What is your theme?

Maybe the children could be involved at this early stage, you could encourage their ideas and help them to realise the excitement of being able to suggest something that is acted upon.

3. What contribution do you want each child to make?

The children could individually prepare their own artwork and the collage be built up by an overseer from their pieces. Or, if you prefer a more integrated effort, then the children can be encouraged to add artistic ideas and materials to a larger canvas or frame, possibly within a guide or outline.

4. Don't forget safety issues

Restrict the paints and glues used to those that are appropriate to the ages of children: you can't go wrong with water-based glues (PVA glue and glue sticks) or water-based paints and varnishes: including some acrylic paints.
Beware of choking hazards with young children: especially beads, small stones or glass pebbles.

5. Protect clothing with aprons, old shirts or even just bin liners with head and arm holes

6. Why not photograph the end result so that each child can have a memento of the event?