Sex is a very important part of a loving relationship and without it there is almost always unhappiness for at least one person. The overtly romantic stuff though is not really the way to go.
Doing your fair share of the house work, cooking, childcare and making sure DW gets plenty of rest and she has a fullfilling life outside the bedroom is a really big help. Sounds mundane but it works better than buying her sexy underwear.
Is your DW unhappy with your relationship, stressed about money or her job, is she ill in some way, has she got thyroid problems, diabetes, is she just exhausted from 24/7 childcare, breastfeeding can reduce libido, being overweight, or even just feeling overweight and ugly, there are many physical and emotional reasons that could be in play. Are there any of these factors that are apparent? Seeing a GP and getting physical and health issues sorted and talking to her openly about emotional issues or even it may be worth considering counselling.
On the practical side. Me and DW found that going to bed early was also a big help rather than leaving it to the end of the day at 11.00 p.m. It leaves a lot more time and we were less tired.
Also, how often are you actually having sex? You say 9 out of 10 times it happens, but if that is every day or every other day that may be just too much for her. After we have had sex, i find taking sex off the agenda completely for a few days and just enjoying a kiss and cuddle without expectation or pressure is a big help. I also try and compliment DW on what she is wearing and how her hair looks, a litle smile or a kiss and 'you look nice' is quiet a romantic thing to do but there is no price tag attached. She does the same to me.
At the end of the day though, if your DW just does not want sex, she has to be honest about that and allow you to go outside the relationship. It is not fair for one person who has a libido to be imprisoned in a monogomous relationship with someone who has no libido and who is not willing to deal with their low libido.
Incidentally, it is not a solution either for your DW to just endure sex or try and avoid it as much as possible and then do it to just keep the peace. That will make you feel rotten becuase you know she is not enjoying it and she will feel rotten having to do it.