When space is tight try using plastic boxes with lids placed on a shower curtain on the floor. Any spilled sand can be poured back into the box and put away. Try using different material such as rice, birdseed or beans. Scenting water with lemon juice or vanilla extract provides another pleasant opportunity for children to use their senses. Art areas require a table and chairs, so materials may be spread out.
Children learn best by handling materials and practicing the process of creating.
Focus on this and not on producing identical products that look good on refrigerators. Store supplies where children can reach them. This can be a challenge in multi-age settings, but try. Keeping materials within children's reach shows your confidence in their ability to become independent. Art and messy activities should be located in a place where cleanup is easy and the children can help.