Is it getting enough sun? According to bonsaigardening.org it needs full sun with at least 1 hour outside each day. The picture shows it in a dark corner.

If it came from a supermarket or garden centre it may well have be mistreated before you even received it. I know squat about bonsai - except that they don't thrive for me :-(

Have you tried checking out GoYpedia? Go to bottom of page click on letter 'B' and search for bonsai

latin name: Carmona microphylla
common name: Fukien tea
family: Boraginaceae
group: Bonsai
origin: China, Korea, Taiwan
climate: subtropical
minimal winter temperature: 18°C
ideal temperature: 22°C
light: bright
soil: humus-peat-loosely
Flower colour: white
rarity: yes
advice:
sinoym: Ehretia microphylla
A very small genus of tropical tree which was once referred to (and still often listed as) Carmona. The most widely known Ehretia species is the Fukien tea, a tropical shrub originating in Southern China and other parts of Southeast Asia. It can be grown outdoors in warm climates, but is quite popular as an indoor bonsai. Ehretia anacua, a recent addition to bonsai, is more resistant to heat and draught.
Propagat by seed or softwood cuttings in spring or summer. Cuttings root more readily if given bottom heat.

Repot it every 2-3 years, in early spring. Reduce water after root pruning. Bottom heat helps stimulate new root growth. Use basic soil mix.

FUKIEN TEA (Carmona microphylla).
Unfortunately your grandsons have bought you a difficult bonsai to take care of (probably throught no fault of their own. Just bad advice from the outlet supplier).
Your bonsai is an indoor bonsai and can only be taken outdoors during very warm summer afternoons/early evenings.
Leaf loss can be attributed to too much watering, too little watering, poor light, too cool, draughts, moving from one location to another, too close to radiators.
A humidity tray can be utilised and this can be beneficial to the tree. A humidity tray is a small tray covered with small pebbles or grit and then filled with water. The evaporating water keeps the air surrounding the bonsai damp.

To check to see if your tree needs watering push a wooden barbeque skewer into the growing medium amongst the roots, leave it there for five minutes then pull it out. If the skewer is damp don't water the tree. If it's dry, water the tree until water flows freely through the bottom of the pots drainage holes.
I hope this helps.

here here tongue

there is so much mis information about bonzi trees and gardening .well real everything .