The trouble with sand eels is that they can be hard to find.
If you are lucky and locate a shoal they can be caught on sabiki rigs with very small hooks, size 14 or less because sand eels have a very small mouth.
The rig should be fished over a sandy bottom or not far from one since the sand eel prefer sand bottom where it hides by digging a shelter. Fishing close to the bottom is often the key to success, but often the sand eel will feed higher in the water during warmer spells.
Professional bait suppliers use a seine net or push nets and special sandeel traps, have a look on eBay or google for an image.
The kids might even enjoy hunting them with one of these kiddies fishing nets, hours of fun - in the right conditions obviously

The closed season for bass ends on June 15th at midnight, which gives you ample time to practice catching sand eels as bait. Hope that helps.