The first thing you need to do after you designate them a desk and an area in the house, is to hang pictures of the collections that they are looking to pursue.
You can get posters made, or even get someone to draw things that are related to whatever it is that they are interested in. This will get the kids to focus and stay in their area, since they can connect with their favorite things.
Make small displays, either by using jars, picture frames, or boxes to display the more unique and rare items that the kids have collected. This will allow them to constantly look at their accomplishments and strive to do even better in the future.
However, this is also a great way for them to build for the future, since they will realize that if they do something good, they can display it and learn from it.
Shelves and drawers
Another way to make the collectors space more appealing is to add shelves and drawers to the desk. This is because with the extra storage space, they will be able to add more items to their collection, which will help it grow and develop into something far bigger than they would have expected it to.
Adding drawers is also a good idea, since they can store items for immediate access over there. The shelves would store items for longer periods of times, but the drawers would be there for a much quicker access to things.