Welcome to the forum.
Some people use "just now" to mean "now" or "right now". In BrE, it generally means "a few moments ago". On that basis, I would use the past tense with it. "The boys were in the shop just now".
With your second example, it depends whether it is still morning. If it is still morning and your mother is still currently making the cake or hasn't started it yet but will start it before noon, then you can say "My mother is baking a cake this morning" (note that we sometimes use the present tense to denote the future). If it is now afternoon and your mother finished baking the cake this morning, then "My mother was baking a cake this morning" is possible. "My mother baked a cake this morning" is better.
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