I have been using the doman method with my 4 month, 2 yr old, and 3 (almost 4) year old for over a month. I have created several powerpoints (20+) with ten pictures and ten words per presentation. The range from different topics of color, mammals, reptiles, birds, shapes, math (doman dot cards), emotions, family, body parts, actions, and 1 for their Spanish lesson based off the free SALSA videos that have a full free curriculum online.
How do I know when its too much? The children seem to never get bored of the presentations, but are they not absorbing as much because there are so many presentations? Are they capable of taking in all that information?
I was going to throw in encyclopedic knowledge presentations but I am afraid of overloading them. Thanks for any help.
Parents in general are the best judge in gauging their children’s progress, interest, and overall balance in life. How much time do you spend on it in relation to time spent reading books, playing outside, doing crafts, etc etc. I am guessing very little! If they are having fun, just go with it 
And when you get tired of making your own curriculum, look into Little Reader!
It changes day to day. Some days my kids will/would sit for 45 minutes strait happily soaking in everything I showed them, then some days I am lucky to show them a set of ten words in fast flash! If you can engage them and get their attention and you are keeping their attention then keep showing them. If their attention wanders they have had enough. So stop. You will find that your oldest will probably happily learn for longer than your younger kids. Also if they are relaxed ( after bath, before bed) in a quiet mood ( yes they are rare ) then you can usually have more success than when they want to go outside and play.
Overall it would be a struggle to overload a toddler or preschooler but it could happen to a young baby. I can recall a number of occasions when my first was overwhelmed by stimulus at the shopping centers up until about age 1. Remember not to interfere with their regular nap times or eating times. You are more likely to under teach than over teach Oh and yes they truly can soak it all up! Amazing hey!
Big thanks for recommending the SALSA program to learn Spanish, what a great resource! Karma to you!!