One idea for developing estimating skills in children, which I saw during my teaching practice is as follows:

You need several A4 pieces of papers. On each of them print a different number of objects, i.e. bugs, buttons, cars, etc.

Start by using the same kind of objects on one page, later you can increase the challenge by adding two different kind of objects, etc. The number of objects in each page should be no more than the number children have reached in their counting skills, e.g. 10, 20, etc.

Show the children one page as a flaschcard, children see page. Turn page over, and the children have to estimate how many objects were in the page. Turn page over and check who was the most accurate.

Increase speed in flashing the page as children develop these skills.