I really think that it is too soon to be thinking about tacking and going upwind. These very small rigs don't provide enough mast foot pressure anyway. Tacking is a chore. Little kids need to get time on the water and enjoyment to persist with windsurfing. I would make life easy for them. I have now taught two young kids and feel that a centre fin is just not necessary. When they first start, they should always learn to point off the wind to get going and so avoid getting pulled forward or the sail getting pulled out of their hands. Centre fins will make the board round up into wind making the sail much harder to hold onto. As they develop stronger sail control, and they graduate to a sail that actually has some mast foot pressure they can then start learning to tack and upwind sailing. I have a video of my 5 year old daughter learning to windsurf in Bonaire. The kids there are all taught to gybe only. You can see some great gybing from her on her first few days. There is also a tack in the video but you can see how difficult it is for her to achieve a turn this way with her 1 metre rig. The board took a long time to turn and then she had to push it around with her feet. The gybes were fast and effortless and allowed her to get a feel for what she was doing. I have definitely changed my ideas about teaching kids and now honestly think that it would be the wrong way to go using a centre fin. It does of course mean a lot of walking of the board upwind. It is worth it though because if they don't get a feeling of accomplishment they won't want to keep going. It's all over then!
For evlPanda jnr: The board will be perfect - I agree with the fin size as Gestalt advised (10-12 cm). You won't need 110 kids board for a while (maybe next year). Teach him to uphaul, reach over and across, step back behind the mast and then sheet in. When in trouble let go with the back hand. Practice this drill at home to get it into muscle memory before going on the water. Make sure he knows all the terms and instructions that you are going to use on the water: Sheet in/sheet out, mast forward/back, step back, front/back hand, front of the board/back of the board. To do a gybe get him to hold the uphaul and lean the mast forward and help turn the board with his feet. Do lots on the board at home before going out on the water. This increases his chances of instant success!