First off, I am not a step-parent but I’m hopeful that this would be a good place to get advice from people who have been in similar situations previously.
I have been with my DP for about three years; he has two DC who are 5 and 6. I have no children. We have intentionally taken things quite slowly with regards to me seeing his DC - we were both keen to ensure that we saw a long-term future and didn’t introduce me too early. I first met the DC around a year ago, and we have built up the time I have spent with them slowly - starting off with meeting up in the park etc. for like an hour. (I will also add that the relationship between my DP and his ex-wife is reasonably strained - she was really opposed to me being 'allowed' to meet the DC at all and DP has historically been worried about her withholding contact. I am not saying this at all to paint her in a negative light, but feel it is important to give the full picture).
In terms of how things are now - they are going really well! I spend a lot of time with the DC when they are with my partner (he currently has them for one overnight per week and a couple of afternoons mid-week, with more frequent time during the holidays; he would like this to increase in future) and we get on really well. They seem happy and secure with me being around and ask if I can come over if I am not there. They have recently (in the last couple of weeks) started asking about sleepovers and asked if I can sleepover at their Dad’s house when they are there; I currently have never done this.
DP and I would like to move in together in hopefully the not-too-distant future - obviously, for this to happen we would want for me to stay over when he has the DC. To both of us, this feels like quite a big step and I don’t know if I am overthinking everything or if I am just being understandably cautious! Is there any point in me staying over but in spare room at first or just spending the night in DP’s bed? What should the message to his DC be? Also very conscious that DP’s ex-wife will not be happy with this development - I don’t think there’s a particular solution to this but it is still a consideration.