As explained under Jacob 5:24, there are a few instances of “behold that” in the original text. What we have to remember in these cases, as here in Moroni 8:22, is that behold can be a regular verb in the Book of Mormon. Moreover, it can mean ‘note, observe’ as well as ‘see, look’; that is, the meaning often refers more to knowing than to visually perceiving. Here in Moroni 8:22, we have an imperative form of this verb with the meaning ‘note, observe’. On the other hand, modern English speakers tend to interpret the imperative behold here as meaning ‘see, look’; and under that interpretation, the use of that is odd, even ungrammatical. In other words, we expect a comma after behold and no following that, thus “for behold, all little children are alive in Christ”. Yet with the meaning ‘note, observe’, the that is perfectly acceptable, as if the text read “for observe that all little children are alive in Christ”.
Elsewhere we have indicative and infinitival uses of behold with the meaning ‘note, observe’, as in these examples:
The second example is particularly interesting in that Mormon is speaking to us as readers of his book—and we are not present to actually see Christ teaching the people. Even in the first example, Samuel the Lamanite is speaking to the Nephites and telling them that the Lamanites (who do not live among them) are living righteously. Thus in these examples, the sense of knowing seems much more pertinent than the sense of visually perceiving. In such cases, the use of the that is possible. For the same reason, the use of “behold that” in Moroni 8:22 works (as in a few other passages, listed under Jacob 5:24); in all these cases, behold refers to the conveying of information rather than actually seeing.
Summary: Retain the that after the imperative behold in Moroni 8:22 since the meaning of the verb behold here is ‘note, observe’ and refers to knowledge rather than visual perception.