What is the best & easiest way to control a big family ?

3-5 is not a big household. With big households 7-8 I pause the game when I need to give orders to several Sims at once. I usually queue up a list of tasks. Example: Go use the toilet, take a shower/Bath, do homework and get something to eat. Have two refrigerators so if one is blocked they will use the other. The same goes for the Stove.

You could opt to make them happy with cheats.

How I did it for a family of 2 adults Sims and 6 kids:

I would have one Sim make big meals, and every morning we would go through a routine of toilet, shower, eat, watch TV.

All did different things so they wouldn't get in each others way. At night we would go though the same routine, just added homework and sometimes had them work on skills. 

I did this until they all grew up. However we didn't have toddlers at that time, which are pretty hard to control in my opinion. 

It's mostly routine, doing the same thing every day.

If you don't mind mods, i highly recommend using MC command center. When i have large families of 6 or 7 i set the game speed slower so i don't feel so rushed to get everything done and it's easier to raise toddlers that way as well. If you're not into mods, like what jpkarlsen said, pausing and filling up everyone's Que with tasks before going back to live mode helps a lot. 

Another way i play sometimes is i choose what sim i want to control for the day. I just let the other sims do whatever they want with full autonomy. Having the lifespan on long i feel helps with it because you don't feel rushed to get everyone's whims/aspirations completed. You can really take your time and experience the game more than worrying about everyone's needs. This way you can also afford to not control all your sims for one day because they'll have plenty of time to catch up on whatever progress you want to make with them.. if that makes sense? 

other mods that help me are the go explore mod by KawaiiStacie and social activities by LittleMsSam. These provide rabbit hole activities to send your sims to if you want a break from playing them. Also, UI cheats extension is incredibly helpful if you don't care about cheating needs. I don't mind cheating when i have a big family, it allows be to experience more game play rather than sending a whole bunch of sims to the toilet all day.