I went kitesurfing in the Grenadines for 10 days at the beginning of January in sailing / kitesurfing trip organised by Planet'Air Kite School (planetair-kiteschool.com). Amazing kitesurfing! Great wind (between 14 to 30 knots every day except one), wonderful spots (Caribbean blue water, nice sand, dream islands), great team.
Tobago Cays is the most amazing spot of the Grenadines. The islands are only accessible by boat, there is no hotel / accommodation. Jameson Island and Petit Tabac are apparently the only places allowed by the rangers to practice kite surfing. I only went to Jameson Island. You go through all the shades of blue in the water, really incredible. A great place also for snorkelling with a lot of turtles and rays. Mayreau is beautiful with the best spot in Saltwhistle Bay. Union Island (right next to the Airport) is also a great place with a nice kite school on the spot.
Except Union Island, these islands are not really spots for beginners because the beaches are quite narrow, trees are very close and shores are often rocky.
I did a short video of the trip...
Kite surf in Tobago Cays / Mayreau and great lobsters in the Tobago Cays...
Planet'Air was great. The boat is beautiful with 3 double cabins. The crew is formed by a skipper, Franck, who is also a kitesurf instructor + an amazing cook, and a kitesurf instructor, Romain (speaking good English). You decide where you want to go, which island, which spot and the boat will take you there. The trip is good for people who wants to make good progress in technique or learning new tricks. Romain is really a very good instructor and he makes good use of the video to make you progress. The boat will take from 4 to 6 people. Good to organise with friends or you can join other people on the people (like I did).
Some tips: If you go to the Grenadines, if you are not a huge fan of sailing and / or you want to maximise you kite surf time, fly as close as possible to the Grenadines (Barbados or Grenada) and ask your boat to pick you up there, or take a regional airline to fly or a fast ferry to Union Island (very conveniently placed to travel around all the islands around). The navigation from Martinique is two (long) days and the return is... really bad... going up-wind. After a bad experience some years ago, I decided to go back to Martinique by plane and it was a great idea! About the kites, I took a 12 Cabrinha switchblade and a 9 sqm Cabrinha nomad. I used both.
Overall, one of the best places if not the best I went for kitesurfing. Enjoy!