I’m like you OP (including twins) and joined a gym a couple of years back, thinking that I could work my abs back to life now I’d had my babies.
I also have a really annoying lower back injury from my twin delivery that everyone said would benefit from strengthening my core.
I gave it my all, I really did - weights, personal trainer, Pilates, yoga plus an hour cardio stuff that was called ‘warm up’ or ‘cool down’.
I walk everywhere, have a physically active life and we eat very cleanly.
While I lost weight and was a little less wobbly on my stomach, all the extra faff and cost made not much difference to my post DC tummy and not a jot to my back pain.
Cancelled the gym membership this summer. I stopped going 6 months ago.
Merrily wearing a bikini (I looked just as good as before the gym) on holiday and playing with the DC and DH.
I don’t look like I did in my twenties but then, why the fuck should I 
My takeaway is that you can see improvements if you have a sedentary lifestyle and a shit diet or you have an active lifestyle, good diet and the time to work out really hard every other day.
I’d honestly rather have a good relationships with the people I love and look slightly raddled in a bikini 