The Shemirah Shabbath by Rav Yehoshua Neuwirth discusses games and toys in Chapter 16. The closest thing to a crafts which he mentions as permissible for children under bar-bat mitzvah age are
(18) Toy building blocks and
(30) blowing bubbles.
In both cases, he seems to say that it is not permissible for an adult to encourage children to do so. Rather if the children are playing with them, the adults are not required to stop the children. See quotes below.
Therefore, it would be inappropriate to lead children in a formal group.
(18) a. One need not stop children from playing with ordinary toy building blocks or interconnecting blocks of any kind, unless they have to be screwed together or very tightly fixed together, in which case they may not be used.
(30) although adults should not blow soap bubbles, one need not stop children from doing so.
Throughout The chapter he mentions other examples which are fully forbidden. For example,
(4) a. One need not prevent children from playing with sand, provided the sand is of a fine consistency is dry and was prepared for this use before Shabbos as in a sandbox. [But] b. Is not permitted to mix the sand with water or to pour water over it, as this contravenes the prohibition against kneading.
(13) it is forbidden a. to shape models out of plasticine, Clay, wax or the like and b. To pour plaster into any kind of mold.
(19) one should not make boats, hats or other objects by folding paper.
(20) it is not permitted to assemble or dismantle a toy from a model making kit, such as a model airplane or ship made from plastic, metal or wooden parts, as the parts join together with a great deal of precision and they are normally built with the intention of leaving them assembled for a considerable time.
(28) All games involving cutting or sticking, whether with glue or with adhesive tape are prohibited.
(36) Plaiting or weaving together threads, strands or strips of plastic or other material is prohibited one Shabbos and YomTov.
Throughout the chapter he suggests various games, such as
(5) arbles, only on floors not bare earth or ground outside.
(6) Table tennis or ball games inside or out with an Eruv but not on ground, only on hard surfaces;
(11) Playing "5 avanim" (an Israeli game normally played with a set of five blocks but could be played with stones, as long as the stones were set aside for the game before Shabbos or YomTov .)
(23) One is allowed to play games in which letters, or parts of letters or of a picture, are placed side-by-side so as to make up a whole word or picture provided that this does not involve setting the word or picture in a frame that holds it together and the various sections are not interlocked and fixed together as most jigsaw puzzles.
(16) Playing with a game consisting of (usually) 15 movable letters or numbered squares set in a framed board the size of 16 such squares and rearranging the squares by moving them about within the board, is permissible on Shabbos and YomTov.
(33) Dice Games, for example "Chutes and Ladders" are permitted, so long, of course, as they do not involve any for bidden activity.
(34) a. Games like chess, Dominos and "Fish" (a card game in which for cards are dealt to each player and have to be made into sets) are allowed.
b. However care should be taken when play has ended not to separate the pieces or cards by color or type as this contravenes the prohibition against selection.
c. Pieces or cards may be separated and sorted with a view to playing with them right away.
(39) a. Running and jumping games, such as tag, hide and seek and skipping with a rope are permitted, but
b. The performance of physical exercises is prohibited as detailed in chapter 14.
Of course, other activities as noted by some of the other answers are also educational and/or fun for children.