Expose your child regularly to French. Play a French radio station, TV station or DVD. Children's programming will engage him with its familiar animation, and the spoken French will be geared towards young people and will be more basic. Speak French yourself if you know how. If not, listen to the French programming along with your child so you can learn it together. Make French exposure a regular, predictable part of your routine. For example, have the French radio station on every morning while you make breakfast, and play a French cartoon every day before you watch the evening news on TV.
Show your child the similarities between French and English. Many words are spelled exactly the same with the same definition in both languages, such as table and orange. Many others are very similar, such as blue and bleu. Some words are spelled the same but have different pronunciations and definitions, such as "chat" which means "cat" in French and "talk" in English. Still others are variations of a similar word with the same definition, such as "finish" and "fin".
Read French children's books to your child. Expose her to French in print the way you would with English. Incorporate a French book into your regular reading routine: if you usually read two stories at bedtime, choose one that is in French and one in English instead of reading both in English.
Start building your child's vocabulary by introducing words in French that are different than in English. Introduce one word each day, or more if your child is willing, and use that word in conversation. Instead of saying "sit on the chair" say "sit on la chaise". Use "open la porte" instead of "open the door". Expand to entire short sentences when your child is ready.
Use French phrases instead of English. Choose phrases you would say often, such as "take my hand." Instead say "prendre ma main." "Wash your hands" becomes "se laver les mains." Say "venir ici" instead of "come here." The more common phrases such as these that you can use, the more comfortable your child will be with French.
Learn French songs together. Children love to sing. Translate the lyric for your child so he will know what he is singing about, and then have fun singing together.
Schedule a French-only hour each day. During that time, try your best to use only French with your child. Be careful not overwhelm or frustrate your child though, as this can make her resistant to learning more French. If your child is not willing, then learning is not likely to happen, so follow the lead of your child's mood or level of interest when deciding how much French to use at one time.