£9 for a pack of 24 realistic looking plastic eggs on Amazon.
Buy some Sharpies and maybe some stickers and you can decorate them yourself or even with the children before you "send them to Easter Bunny to hide".
Better yet, you can reuse them for next year, let the children take them home if you want to make a tradition of decorating eggs or even do a mix and match where you can put an equal number of eggs that each child has decorated over the years and task them with finding only their own eggs.
That last one is good if you want to include their names or initials so they and you can later identify who decorated which eggs.
It can also be a good way for you to introduce co operation among younger children and remove rivalry.
