Exactly the same boat, but we're coming out of it.
Most important thing is talking because it has to be solved by both of you. You need to find out why her libido has gone.
With us it was a combination:
1) She felt unattractive. This took a lot of persuading that I find her attractive even though my wife isn't fat by any means. Me finding her attractive was only the first step though and so diet and exercise was required. This is really tricky because if I suggested diet and/or exercise she took it to mean that I thought she was fat. What worked for us was the 5:2 diet which I can also do for the health benefits. Also I want to get fitter for surfing and snowboarding so we're both on a fitness regime. The approach that worked was "I want to do X because I want to be fitter/healthier, do you want to do it with me?"
2) Day to day stress. Remove as much stress as possible. Do the washing up/tidy/whatever so that once the kids are in bed she can truly relax.
3) Tiredness. Let her have lie-ins at the weekend. That way there's no rush for sleep on Friday and Saturday night.
4) Affection. She'd started to think that if I showed her affection, I wanted sex. Lots more non-sexual affection.
5) Lack of "us time". You've said this is difficult, but we share babysitting duties with other parents. Also we have date nights at home with a take away (saves washing up), no TV and chat and play (non sexual) games. On a date night don't expect sex. Let her make the moves and if it doesn't happen it will still reap benefits in the long term. If you possibly can try to arrange a weekend away for the two of you. We do this about once a year and it reminds us what it was like before kids.
6) Monthly cycle. This has had an increasing effect after kids. The week before her period there's no chance of sex. The two weeks after there's a much higher chance. Don't make any advances in the unlikely weeks, but keep up all romance affection etc. etc.
7) Romance. Gifts, flowers, compliments, etc. all make her feel better.
8) Other factors. If I hadn't had a shower or shave; if I was wearing underwear she didn't like; if the bedroom was untidy or the bed wasn't made. I try to remove any factor that gets her out of a sexy mood.
This is just the things that apply to us and you might well have a completely different set. The key thing is not to expect sex. Do all the things and avoid pestering because pestering ruins all the good work.
A big plus is that things are improving for us.As our affection levels have increased, and she's getting fitter and more body confident, her libido is going up and up.