Its the sefer kitzur shilah (printed several hundred years ago) who actually states that these pesukim should not be said by adults. Indeed they are not found in most of the older siddurim. (Interestingly, modeh ani itself, which is brought down in the poskim, did not make it into the siddur until the late 1500s, when its nussach was authored. Other versions were composed in later siddurim, but the original one was adopted by most siddurim. Although the yerushalmi has a tefilla that shares the beginning of modeh ani it has little resemblance to the modeh ani that was introduced to siddurim a little over 400 years ago.). The siddur Ateres Yerushalayim says that there are those who are opposed to saying ראשית חכמה etc. since one should not say pessukim prior to bircas hatorah, which presumably is why the kitzur shilah was opposed to it as well.
Thanx. The kitzur shlah quotes the posuk torah tzivah, which is really a gemara, where did reishis chochma and shema bni come from?
Author of modeh ani is the seder hayom, lived in safed about 400 years ago. It's quoted in magen Avraham siman 4, there's another version from the yerushalmi quoted in the first biur halacha.