well, if Qasim, he'd be 25-30 years old when Muhammad dies, without a doubt he'd have the unconditional support of Ali and Fatimah's otl supporters, most likely Qasim would have married Aisha, and not Muhammad.
Basically he'd be uncontested since Abu-Bakr wouldn't contest against his grown Son-in-Law.
Ibrahim, the last child of Muhammad would on the other hand be three years old.
Of course once the original line dies out (no sons or none survive to adulthood) the power struggle will be ferocious. You may also see the first son, when he takes power, having his brothers done in like Turkish sultans did. Multiple legal wives as well as concubines makes the line of succession potentially quite murky (see how Saudi Arabia does it for example).
Well considering that the kings of Jordan are directly descended from one of Fatima's sons, that could take awhile. fratricide is unlikely in full brother, it's half-brothers with zero filial piety you see that in, also the turkish system was completely stupid for the purposes of filial piety, and arabic custom holds to primogeniture so unless two of the wives give birth on the same day, well the sucession would be fairly clear at least for the first few generations.