What you describe is completely acceptable
The circumstance you describe is completely acceptable. First, one of the goals of child support is to provide a similar standard of living in each household, so spending an amount on rent that happens to be a large portion of the child support amount is understandable.
Second, the fact that the child only spends part of their time at Parent B's home is already accounted for in the computation of the amount owing. Child support in Canada is based almost exclusively on the income of the parents, taking into account the proportion of time the child spends with each parent. And this is all subject to any specific hardships of the paying parent.
Only where a portion of child support is based on a special and extraordinary expenditure (e.g. a specific extracurricular activity, or medical insurance) is it expected the claimant make that specific expenditure. If they stop making that expenditure, then the child support add-on associated with that expenditure is removed.
* I understand the question is directed to Arkansas, but I have provided an answer for those interested in Canadian law, in line with the guidance here ("Even if you supply a jurisdiction tag, we expect and encourage answers dealing with other jurisdictions – while it might not answer your question directly, your question will be here for others who may be from those jurisdictions. If you do this, please tag your answer using the tag markdown: [tag: some-tag]").