Is it permissible to combine the monthly allowances and monetary gifts designated for the children and the mother, then divide them equally among all children in the future, or do the aforementioned amounts become rightfully owned by those for whom they were initially designated?

If Allah blesses you with more children after your firstborn, it is better to reclaim what you gifted to your firstborn and make the gift for all your children equitably, by distributing the amounts equally among them. This equity is specific to the children and does not include the wife. If you designated a sum of money for your wife and she took possession of it, then it belongs to her. As for the money gifted to your son by someone other than you, and you took possession of it on his behalf, it is specific to him and should not be added to the money you intend to divide among your children.