Yes Khun Nattima, It's good if you can do it in front of the others, that's the way to make your child confident to speak out. The hero of the children is their parents and they are such a good copy cat too!!
I agree with K.Orada to speak infront of the others,do it natually. I have been speaking english with my Ja(my daughter) since I made decision to start my lifetime english project with my kid. Please feel free and relax to show your confident to your kid and do lead your lovely kid by example. Be yourself because you didn't do anything wrong. Keep it up.