If you don't know, say, "I don't know."
Or say, "As soon as I know I'll let you know"
If you need time to determine if the reply is 'yes' or 'no',
say "Maybe..." "We'll see..." "It's possible..." "Probably not..." "That's all I know right now" "I have to think about it"
The best honest reply that works in many situations is saying: "I don't feel good about....."
For example, rather than lying, "I don't have any candy." Say, "I don't feel good about eating candy right now." Then follow with some kind of postive, "But you can have a carrot."
State your "no's" clearly, firmly and gently.
Rather than lying, "The amusement park is closed." Say, "I am sorry, No. We are not going there today." Then validate the feelings and the desire, "That's too bad, it's a fun place to go. You really love it there. What do you like the best at that place?"
If you change your mind it's ok, but validate their feelings and apologize.
For exapmle, "I know I told you that you could play with a friend, but I didn't realize how late it was. You can't play now. I am sorry I said you could, I know you really wanted to."
Honesty...we expect it of them, we must be the example.