can you still have a flat stomach after having 4 kids?

I am 29 years old and i have 4 kids. Im still skinny and can’t see my stomach unless i sit down. when I get into my diet and
exercise spells i work on my stomach the most, but it seems
like it just want buge. Should i get a tummy tuck or do you think
i can get a flat somach on my own with lots of hard work?