Here's a peek at one of techniques we use in the Art Studio to make visually textured marks on papers that we use for Eric Carle-inspired collages.
To start, put a little water-based paint (such as tempera) on a tray or plastic plate, roll a toy truck through it, then "drive" it on paper of your choice. For making collage papers, we like using tracing paper, Folia Transparent Paper, lightweight drawing paper or construction paper.
This truck is especially fun because it can turn to make curved marks. You might cover a table with big sheets of paper and let your young artists go to town. On the other hand, if you need to define a workspace you could put a piece of paper in a large tray or rimmed cookie sheet to help contain the marks.
Another fun idea we've tried a few times in our toddler programs is offering a black paper "road" taped to the floor for a playful art experience. In these times I've offered paint in secondary colors (green, orange, and purple) because when they mix all together they create a muddy brown color. If you try this, you might need to add a dab of white to your colors to help them show up on the black paper.