The children could use the place always.

The correct answer is C. The place could always be used by the children.

A. The place can be used by children is incorrect because it does not imply that the children are always using the place.

B. The place is used by children always is incorrect because it does not imply that the children could use the place at any time.

D. The place has been used by children always is incorrect because it does not imply that the children are still using the place.

The sentence “The children could use the place always” means that the children have the ability to use the place at any time. It does not mean that they are always using the place, or that they have always used the place.