My first advice is to rethink your idea of “curriculum” for the preschooler.  Boxed curriculum that we might buy for our older students can be overwhelming for a preschooler.  I hope we can all agree that no preschooler should be sitting at a desk doing schoolwork for hours on end.  Instead, I have found that the Charlotte Mason approach works really well.  Mrs. Dee, my beloved homeschool mentor, told me when I started homeschooling that all I really needed to do was “Read, Read, Read” for the first few years.

As a kind of very simplistic explanation:  Charlotte Mason’s approach was to consume quality living books and also keep a nature journal from lots of exploration in nature.

Personally, I love the plans for both Ambleside Online and Sonlight History as Charlotte Mason guides.  Both of these include a variety of books from historical fiction to biographies, science, etc.  The books are quality reading material.  Most of them you can find at your local library or pick up for cheap at your local used book store.  Sonlight has an amazing reading list for ages 4-5 that are read in small sections so even your youngest, most inattentive students can handle.  Ambleside, on the other hand, does not start until Year 1 or 1st grade and maybe a bit much for smaller kiddos, but if your 3-4-year-old will sit through a chapter, the books are fabulous.  Plus, I really love that Ambleside provides topics for Geography, Science, and Math that should be covered in the first year.

Secular Homeschoolers Beware:  Both Ambleside and Sonlight are Christian based.  It’s easy enough to skip the Bible sections, but some of the books (and I stress that it’s really only a few) are Christian fiction, so you should read the summaries before starting the books.  Typically, for me, I read the summary as I get to the book on the list and if it feels right, I read it.  To date, I have only skipped a mere handful of the books from the Sonlight lists we’ve done and only skipped the Science and Bible parts of Ambleside.

Ok, maybe you want something more than just reading books.  Here are the things I have used with my kids:

ABC Mouse:  I signed up for ABC Mouse when Little Man was 3 and set up accounts for him and the baby.  They really only played on it sporadically, but I love that you can use it on tablets and my phone.  Mobile education is VERY important when you travel a lot.  I think they could have learned from ABC Mouse, but they spent more time “buying” clothes and accessories for their avatar and playing dress-up.

Reading Eggs and Math Seeds:  I signed up for Reading Eggs and Math Seeds for Little Man when he was 4.  The cost is reasonable, especially if you can get in on the deals through Homeschool Buyers Co-op.  The Baby was always trying to play big brother’s games so I eventually got her an account of her own.  These programs have been pretty amazing for helping the kids learn reading and math, but I confess that they preferred the math over the reading usually.  Little Man enjoyed playing but quickly learned how to cheat the system so I had to switch him to another program.

Handwriting Without Tears:  If you’re looking for a complete language arts program for young children, look no further.  HWOT teaches reading, handwriting, spelling, and phonics.  They even have a collection of songs that you can find on YouTube to go with the curriculum.  I happened to have the songs on CD (yeah, I’m old school like that) long before I was even thinking about the education of my youngest 3 so we already had some of the songs memorized.  I don’t know how much they help, but they certainly make writing more fun.

Explode The Code:   This is another complete language arts program for young children.  We have enjoyed ETC because the pictures are fun, but sometimes the pictures aren’t exactly clear as to what they are trying to correspond to.  For example they have a picture of a person and the word is a name (see example below).  But I love that the kiddos can do 1-3 pages relatively quickly and learn so much.  Plus, for the kiddos that want a little extra, you can have them color all of the little pictures to fine-tune those fine motor skills.

Another option that I didn’t explore enough, but I know several families that have are the education sections at Dollar Tree or the book sections at Walmart or Sam’s Club or other discount retailers.  They have some pretty good workbooks and don’t discount the coloring/activity books.  I know when we think “curriculum” we have something more prestigious in mind than the Spider-Man activity book, but you’d be surprised how great these are for early learners.  Not only do they include a lot of your basic motor skills like drawing lines and coloring, but they also cover letter and number recognition, early reading, simple math from shapes to adding single-digit numbers.  Plus they are often made using characters your kids already love like Spider-Man and Bubble Guppies and Mickey Mouse and Disney Princesses.  Plus, they are often very inexpensive so you’ll have plenty of money left over for a lot of field trips to the museums and science centers to really fill your little ones’ minds.