how to prepare for HIM?

Are you planing to do an HIM is September and March?  Or did you mean HM in March?

Either way, if you have two small children your biggest challenge may be scheduling the time needed to prepare.  Running 4 days a week would be good but you need to swim and ride more than once a week.  If time is an issue maybe just run three days a week because you really should be swimming and riding at least three times a week.  I can get by on two swims a week but two bikes a week is a bit slim.

Do you have a trainer?  I find that helps a lot with kids because you can ride without getting someone to watch them.  Just figure out a way to keep them from the area around your trainer.

You also might want to look at the beginner HIM plan here on BT and also hop into the HIM plan forum.  Lots of ideas and support.

Good luck.

You mention going back to work and have a supportive DH...I'd find a way to use your lunch hour to your advantage, maybe even structure it where you come in 30 minutes earlier or stay 30 minutes later to have more time at lunch to get in a good solid workout.  That way you won't be taking time away from the family and be more focused to use that time to its maximum benefit.  If you aren't a morning person become one :-) The hours of 0430-0700 can be used for some good workouts.  There are plenty of creative ways to get the workouts in without sacrificing too much of your real life.

Enjoy the journey...

Your kids are only this age once in your life time.  They're still pretty young, but if there's any way that you can involve them in your work out's you all will benefit. 

I had a couple of new triathletes last year who brought their two boys,  probably 6 and 8 years old, to the track and they ran a little with us, and they played football on the football field while we ran.

Just a thought.  I don't want to discourage you from achieveing your goal of a HIM.  ...but there are good Oly plans that have two rest days/ week.  I think most HIM plans have only one rest day.

You don't mention why you thought your other HIM was bad.  You can do a HIM on less training if you are willing to satisfy yourself with a slower time.

With two kids under four, and a full time job you'll have your hands full.  Perhaps a book on time management.  There are some good time management tips in Fink's Iron Fit book.  (also search the BT forums)

One last thing.  When your kids get to school age, you will be amazed at how fast their time being little passed.  Mine are 22 and 28 years old now.  You only have them to yourself from now till kindergarten.  From then on you share them with the world.  And pretty soon they're more interested in hanging out with friends than with Mom or Dad.

What ever you decide to do, good luck, and enjoy yourself and your kids as much as you possibly can. 

A little bit of a different approach here.  Focus on your running and biking, skip the swim workouts provided you have decent technique all ready, you'll build your endurance back in short order.  If your technique is in question forget all of this.   You have all of next spring/summer to work on swimming.  Hit the pool when time allows and work on drills if you like.  

I never believed Endurance Nations mantra of skip the swimming during the off season (they call it the out season).  You can put all kinds of work in the pool and gain a few minutes at most in a tri, but by putting more effort in the run and bike and you can save even more time and finish the longer races stronger. 

I found this to be true last year when I had a bad crash ended my year in June and had a total separation (grade 5) and I was unable to swim for 4 months and out of work for 5, once I learned how to maneuver my right arm again to prevent pain it didn't take long before getting my swimming back.  After having all that time off I was easily swimming just as well 2 months later even with a shoulder that isn't 100%.