Well, the Bible says "covet earnestly the best gifts". A little out of context, huh?
I did a KJV word search on "covet*" and 1 Cor. 12:31 & 14:39 were the only 2 places I could find where coveting didn't have a negative meaning. After looking at the 39 verses that contain a form of the word, I think that coveting (desiring) toys is OK as long as they're not desiring their neighbor's toys or the wrong kinds of toys.
Here are a couple verses that might set your mind at ease:
Charge them that are rich in this world, that they be not highminded, nor trust in uncertain riches, but in the living God,
who giveth us richly all things to enjoy; 1 Tim 6:17
Behold that which I have seen: it is good and comely for one to eat and to drink, and to enjoy the good of all his labour that he taketh under the sun all the days of his life, which God giveth him: for it is his portion. Every man also to whom God hath given riches and wealth, and hath given him power to eat thereof, and to take his portion, and to rejoice in his labour; this is the gift of God. Ecc 5:18,19
Since your 4 year old is a little too young to rejoice in his own labor, maybe he can rejoice in mom's & dad's labor. ;-)