fuse beads; perler beads
Small plastic beads placed on a pegboard and melted together with an iron to create designs. A wasei-eigo term; in English they are often called 'fuse beads' or 'perler beads' (a brand name).
wasei-eigo (Japanese-coined English), from iron beads
See also: アイロン
I made coasters with fuse beads together with my child.
You can buy fuse beads even at 100-yen shops.