Listened to the news report on Philadelphia's KYW newsradio (103.9), which featured the press conference with the district attorney.
Poor DA Steele was audibly distressed as he described the charges, and choked up several times as he explained that he himself is a father of a 12-year old, and could not fathom someone doing such a horrible thing to their child as to strangle them to death with their father's belt.
The alleged motive was that the mother wanted to spare the child from his worries about the family's finances--yet the child was enrolled (as others have noted) at an extremely expensive and well-respected private school, the mother drove a 2019 (so new-ish) vehicle, and ostensibly the family's home was large enough that the mother could leave it without being seen or heard by the father.
It seems more likely to me that the parents were about to split up--the father was sleeping in another bedroom, which (per the father's comments in the arrest affidavit) was an ongoing arrangement in the home--and this would've caused there to be more talk about money/finances/alimony/etc. that the child could overhear. Because this is just my conjecture, I don't know which parent is more likely to have initiated a split (if there was, in fact, to be a split). However, if the father had been the first to raise the issue of separation or divorce, it would support my guess that the mother killed the poor child in order to maximally hurt her spouse, rather than for any altruistic reason.