I'm enjoying this! Like one of the other people in the comments, I also think you should make maybe 3 words available on the first level of each new type, and upgrading adds 1 more word every say 2-3 upgrades, to a plateau of maybe 10-20 words per category.
(Also, the animal LINX should be spelled LYNX for the large cat. If we're spelling words, let's not accidentally learn incorrect spelling through repetition!)
On reflection, having got through to vehicles, I now have another potential idea. What if you had a 'teaching mode', where you could input a child's list of 20 spelling words into a level, so they could practice these? It would need to be able to spellcheck them I guess.