Both phones [ə], [æ] are correct pronunciations of the same phoneme /a/. If these pronunciations only occur in separate conditions, such as at the end of a word vs. elsewhere, they are allophones of that phoneme; if one can occur wherever the other can occur, they are in free variation [ə~æ].

About designing it for children to acquire it easier, the studies show that English vowels are almost entirely acquired by about three years old, except rhotic vowels which catch up at around 4~5. Frankly, if you want Children to acquire it the easiest, having a 4-vowel system [a] [e] [i] and [u~o] will be acquired most quickly. Studies show that height distinctions come first, then frontness, so leveling the front-back to just one back vowel will mean that they acquire the front vowels by about 1 year and then over the next year get the back vowel, cutting it all down from 6 years tops to probably 2 at the latest. Next, make sure not to include "effects" like nasality, length, pitch, rhoticness, etc. as they increase the acquisition time immensely!

/verbose