My husband is also a retained FF. Everything Evergreen says about rotas, cover, low pay, being constantly on call and covering for paramedics is true.
My husband was also on call 24/7 when we met. However, when our multiple children came along, this did have to change. The difficulty comes when you can't leave the child alone with them for more than a couple of minutes to pop to the shop in case a call comes in. They can't do the school run in case a call comes in. They can't take the child out on errands in case a call comes in. You have to stay within four minutes of the bloody station all weekend. The alerter goes off in the night and wakes the baby that you've spent hours getting to sleep.... and on it goes.
The solution we came to was for my husband to cut his hours. Despite what the brigade wants, and publicises, there are the means for them to offer contracts with less hours. My husband now has a 100-hour per week on call contract instead of a 120-hour contract (on top of his full time job, although he works locally for himself so can remain on call during working hours) and there are people on the station who only have 40 hour contracts.
Most brigades struggle (especially rurallly) to recruit retained firefighters, so if your husband tells them he can only offer 60 hours, they may well go for it as an alternative to him quitting. If they don't, then I would seriously tell him that he hours he is doing are going to be incompatible with having a young family. There's nothing wrong with that, even if he was a FF when you met. We all have to make compromises when we have children, and this may well be his.
TBH he would probably make more money picking up a few shifts in the local pub. My husband was on call for 400 hours last month, went to four drill nights and earned £270. The pay is seriously abominable, unless you're at a busy station (we're not. Lots of hours on call, very few actual call-outs)