I don't think the porn on browser issue is a huge thing, because on phones the browser tends to keep whatever you were viewing especially if you forget to close the tab. If the toddler is playing on games then it's likely she was semi-supervising, even if not actually watching the screen, you can hear toddler type games and usually if you heard the music or sounds stop then you'd check in case they had managed to navigate onto the phone part and were making calls or something. Obviously it's careless, but I think it would be more that she wouldn't expect a child to tap on a (quite boring looking) icon of a web browser. It's not the same as, for example, viewing porn on a computer screen which could then be turned on to put the cbeebies website on.
I think the suggestion to say "Hey, I saw this, is it something you'd be interested in watching together?" is a good one. It's not saying "I'm upset that you're choosing this over me" (even if you are) and it's not saying "Hey, let's have sex RIGHT NOW", it's just an option to explore together.
If I can be frank, masturbation for a woman is totally different to sex, especially post-childbirth sex, because you are only really touching a very small, outside part of the anatomy and there is no chance that you yourself will either slip and touch something painful or try to explore to take it further. There is quite a lot of pressure generally - not necessarily coming from you - that sex means penis-in-vagina (sometimes shortened to PIV which I will use for the sake of brevity) and/or some other kind of penetration. And although this of course can be and often is pleasurable, it can also feel very invasive, it can be uncomfortable or even painful, even though at the same time it is enjoyable/pleasurable. What I'm saying is that sometimes it's a trade off. The problem is that after childbirth especially, the pain and discomfort can be heightened and the thought of it can be enough to put you off despite enjoying the nice feelings which come with it as well.
It may be that she feels guilty doing other things with you if she is not feeling up to PIV or not wanting PIV at the moment, because she associates this with being "full sex" and feels that if she were to start other things but not progress to PIV then she is somehow being unfair to you or being a "pricktease". She may feel that you are feeling rejected due to the lack of PIV and think that if she was to do other things but not PIV then that would be seen as even more of a rejection by you. She may not know how to explain this or put this into words without it coming across that she doesn't want to have sex with you, or making you feel like you have done something wrong or are hurting her when you have sex (PIV). Also, it may take longer physically for you to reach orgasm together without PIV and she might find the thought tiring or exhausting, especially with two small children clamouring for her touch most of the day - it is extremely common to feel "touched out" and the thought of spending 20 minutes doing a largely repetitive movement is not a hugely attractive one. (Sorry to say.) Mutual masturbation/69 may be more attractive but she would have to trust you not to again, push boundaries that are uncomfortable for her currently. I am sure that you wouldn't be pushing boundaries anyway, but she might fear that you would do so unintentionally or with good intentions.
Bottom line, you need to communicate, she needs to feel reassured that you are not wanting to do anything before she is ready and you need to feel reassured that she does still have the intimacy with you that you always had. The conversation about realising you hardly ever hold hands is a good one - maybe you could try to rebuild some non-sexual, low pressure intimacy where the focus is not on sex or PIV at all but just about touching, being close, perhaps a massage or a bath together or just cuddling close while watching TV or something like that.
Good luck.