If you are booking 4 seats together then the computer should allocate you 4 seats together. So you don't book 4 individual tickets- you book them as a batch- 1 adult, 1 senior (assuming they have a senior railcard) and 2 children. If you book as 4 individual tickets you will just be randomly assigned 4 seats.
But the computer can equally do odd things with reservations.
On an Edinburgh to Glasgow journey no seats are assigned anyway. That is just pot luck. But outside peak times the trains are long and frequent enough that is not an issue.
There are always lots of unbooked seats on the Oban trains (for the many people who buy on the day), especially off season. And boarding at Oban and Glasgow you board 10 to 15 minutes (maybe more) before departure. So you should be able to get to choose your seats. Joining at Glasgow, if deciding you don't like your assigned seats, check the car labels and ensure you are joining the Oban cars, not the Mallaig cars.