My almost 3 year old has over 40 apps that he has been using for about 8 months now. He started Last Christmas. He was 26 months old. Here is a long list of them starting with the ones he started with, up to where he is now. I hope you find this helpful.
1)English Plus flash cards
2) First Words Delux
3) Amazing First Words
4) Shape O
5) Little Reader ( three and four letter words )
6) Toddler Teasers
7) monkey Preschool Lunchbox
8) Little Speller
9) Dr. Seuss books. ( They are great. Interactive. )
10) Educreations ( a drawing board)
11) Animal Puzzle
12) My First Words
13) Word Wagon
14) Spell
15) Kids Puzzles
16) Caterpillar
17) Montessouri Intro to Math
18) Montessouri Crosswords
19) Write Words
20) Coloring
21) Umi Numbers
22) Teach Me 1st and 2nd grade
23) My Name
24) Kids English
25) Montessouri 100 board
26) Write On
27) Cursive
28) Montessouri United States
29) Montessouri US Capitals
30) Learn the World
31) Spell Music
32) Wonders ( about the Universe)
33) Solar Walk ( the Planets )
34) Stack the States
35) Stack the Countries
36) Spell USA States
37) I Spell Countries
38) Bone Scan

There were a few that I missed here and there, but this should keep your children busy. It's all about what your child may be into right now. For example your child may be into Dinosaurs. I'm sure there are plenty of dinosaur apps. Whatever subject you can think of there is probably an app for it. Good luck and enjoy.

By the way I would love to see some ideas from anyone else out there who has some good apps for children. Thanks in advance.