I have 2, young school age children and a preschooler. I work part-time teaching aerobics at a gym, but also as an accountant at home while my preschooler naps. My neighbor is always walking in the door without being invited. We had out-of-town guests, and 2 of the 4 nights they visited, the neighbor and her husband came in and stayed the whole time. How can I let her know without making an enemy of her that this is not acceptable?

Be honest, tell them and hope for the best.
And if they come to the door, merely don't let them in, explain you are busy at the moment.
*** I do hope you mean they knock at the door, not just "come in"

Lock the front door. If they knock say that you are busy or the floor is wet.

Or as my sister does to stop her sister in law visiting who lives down the road... Pretend your asleep.

If you have guest over just politely say sorry I have guest over tonight who I haven't seen in a while I just want to spend time alone with them. How about another night.

Honestly, I had neighbors like that and I really didn't want to be rude... so I just made sure that I was not, with a smile on my face I told them that we were "entertaining presently with our out of towners and now is not a good time for a visit, we'll see you another time ok?" We do keep our doors locked all the time, thankfully.. but our nightmare neighbors always came over using the gate door between our homes... and came straight into our yard WHERE WE WERE... we did have to put our foot down as they gave us no choice. I don't like to make bad blood between neighbors (in fact I truly fear it actually... ), but this is MY HOME where I have my sanctuary and my need and desire to keep it that way over rode my fear of creating drama and potential upset between my neighbors and myself. I felt like I had to switch to "self preservation-mode" in order to keep my space... you know? I was very polite, make no mistake. I later went over with a pie and thanked them for their understanding... much to my husband's chagrin lol :) It did the trick and smoothed out the rough edges... lol.

Why on earth would you EVER leave your doors unlocked with small children in the home? Keep them closed and locked or you're going to have any and everybody stroll into your home unannounced. When you make it a bit harder for the neighbors maybe they'll get the hint.

Lock the door. If they knock tell them you're naked and busy.

You could try inviting her for coffee, sit down and explain that you have lots to do most of the time and that you really do not have the time for her when she keeps popping in. Explain that this constantly interupts you schedule for the day and that with such a lot to get through during each day, you don't have a lot of spare time. Tell her that you are sure that she will understand and if she could phone before a visit you would sincerely appreciate it.
Tell her that you value her friendship etc...

If this does not work, then you will have to take the point blank way and tell her that you do not want her to come over unless she is invited and that she must not take the liberty of just walking into what is your home after all.

Ok, she may get her feathers ruffled for a while but that will wear off... she needs you I feel more than you need her... go for it