It's not easy to teach a preschooler French, work full-time, take care of your house, cook food, and all of the other millions of things that I'm sure you do. Just know that every bit you do today will really help them later in life. If you can even get the basics down now, by the time they are learning it in school they will be so far ahead of everyone else, that they can focus on the more difficult grammar.
Just keep doing it and make sure you at least to a little a day so they see learning French as part of a routine and something that they will do every day. They might fight a little at first, but after awhile they will really enjoy 'their French time' and they will thank you for it when they are not starting from scratch in College French when it is so much more difficult to pick up a language.
