Can I give the smaller agents who work under me their own login on my portal?

Yes, you can. A white label portal is normally set up so that you sit at the top as the main agency, and you can create separate logins underneath you for the smaller agents who sell through you. Each of them signs in with their own username and password, searches and books inside the same portal, and sees only their own bookings and their own customers, not yours and not each other's.

The useful part is that you stay in control of the money. For each sub agent you can usually set the markup they are allowed to earn, or set your own markup so the price they see already includes what you make on the sale. You can also give them a credit limit or ask them to keep a topped up balance, so a booking only goes through when they have the funds for it. If you would rather keep a closer hold on things, you can ask for tickets to be released only after you approve them.

Be honest with yourself about how far this needs to go, because the answer depends on your plan and on the suppliers behind your portal. Some plans limit how many sub agents you can have, or how many levels deep the structure goes, so an agent sitting under another agent under you may not be possible. A few supplier agreements also restrict who is allowed to see certain fares, so a rate that shows for you may not show for everyone below you.

Before you commit, write down how many sub agents you expect in your first year, whether they should see net rates or only your selling price, and whether you need credit limits from the start, then ask for those points to be confirmed in writing.