I would say that 3-4 weeks in advance is good for a standard (at home) kids party.
I think it depends on the type of party and how many guests you're inviting.
If you're planning a huge hoopla? An earlier invite (like what you're thinking) would be the way to go. If you're planning on renting a bouncy-thing in your back yard or something at a venue that requires pre-payment for a minimum amount of guests? Then, Yes. Send out the invitations now.