I've thought a bit more about this.
We'll assume that this Magnus Eriksson is accepted as king by the Guardians of Scotland - would Magnus stay in Norway with additional Scottish tutors under his father Erik or move to Scotland?
Anyways, Magnus remains King of Norway & Scotland until his death between 30 or 50 (based on his predecessors' ages) thus c1313 or c1333. He likely marries a daughter of Edward I and may affirm him as overlord.
I'll outline the possible permutations of his children 0-3 sons and 0-3 daughters.
0 sons & 0 daughters:
Norway is likely inherited by his uncle Haakon if he dies c1313 or Haakon's grandson Magnus Eriksson (King of Sweden 1320, Lord of Scaneland c1332) if 1333.
Scotland is likely disputed between the Balliols and the Bruces TTL without the oversight of Edward I, so possible civil war.
1 son & 0 daughters:
The Union of Scotland & Norway continues with a Norwegian regency the earlier Magnus dies.
The son may take a Scottish bride (Marjorie Bruce?)
Wars are against Swedish incursions are possible.
2 sons & 0 daughters:
End of the union is possible but who gets Scotland?
Some Norwegian vassals - Iceland, Greenland, Orkneys, Faroes - may go to Scotland depending on the equality of the division and which Kingdom is premier.
The King of Norway is likely to follow tradition and thus marry Haakon's granddaughter Eufemia - giving us butterflies in the Scandinavian royalties if Magnus Eriksson is still deposed.
The King of Scotland is likely to marry a Scottish bride (Marjorie Bruce?).
3 sons & 0 daughters:
End of the union is more likely and division of vassals possible.
Maybe 3 Kingdoms - Norway, Scotland, Iceland.
The 3rd son is a possible vassal (king or not) of the greater Kingdom or both.
The King of Norway is likely to follow tradition and thus marry Haakon's granddaughter Eufemia - giving us butterflies in the Scandinavian royalties if Magnus Eriksson is still deposed.
The King of Scotland is likely to marry a Scottish bride (Marjorie Bruce?).
The 3rd son's marriage depends on realm and age.
The descent of the sons will have interesting repercussions.
0 sons & 1 daughter:
Norway likely goes to Haakon or his grandson Magnus Eriksson.
The daughter is likely Queen of Scotland and could marry Edward Balliol, David Bruce, or the future Edward III depending on age and politics.
An early union between England & Scotland will probably only last until a second son is available or the succession is disputed but it may result in England getting an imperial crown of Britain.
1 son & 1 daughter:
Same as for 1 son & 0 daughters.
Additionally the daughter would be married to secure alliance - Edward Balliol, David Bruce, the future Edward III, or Magnus Eriksson of Sweden.
Her age, marriage, and heirs will have interesting repercussions on future successions.
2 sons & 1 daughter:
Same as for 2 sons & 0 daughters.
Additionally the daughter would be married to secure alliance - Edward Balliol, David Bruce, the future Edward III, or Magnus Eriksson of Sweden - with her cousin Edward III probably the likeliest.
Her age, marriage, and heirs will have interesting repercussions on future successions.
3 sons & 1 daughter:
Same as for 3 sons & 0 daughter.
Additionally the daughter would be married to secure alliance - Edward Balliol, David Bruce, the future Edward III, or Magnus Eriksson of Sweden - with her heirs less important succession-wise.
0 sons & 2 daughters:
The eldest is Queen of Scotland.
Norway is likely inherited by his uncle Haakon if Magnus predeceases him but if not then a Norwegian succession war is quite possible depending on the sons & husbands of Magnus's daughters; with an increasing later impact on the Scottish succession.
1 son & 2 daughters:
As per 1 son & 1 daughter but with a daughter more likely to marry Magnus Eriksson who could try to seize Norway.
The son is likely to have a Scottish bride (perhaps with Norwegian ancestry?).
The relationships of his children and nieces/nephews will greatly affect succession in Britain and Scandinavia.
2 sons & 2 daughters:
As per 2 sons & 1 daughter.
Marriage of the daughters will be split between Scandinavian and British alliances.
3 sons & 2 daughters:
As per 3 sons & 1 daughter.
Marriage of the daughters will be split between Scandinavian and British alliances.
0-3 sons & 3 daughters:
Essentially the same as for 0-3 sons & daughters.