Hi Chryss,
You may have read elsewhere on this site, that MY wife, Myrna was granted a 'Multiple Entry' Schengen Visa FREE OF CHARGE to enable she and I to visit France in March. It was issued by the French Consulate in Edinburgh ... which is closest to where WE stay. Presumably [for *YOUR purpose] the nearest *SPANISH Embassy to where you and Adam are living in Suffolk, is likely to be in London ... but check with the internet and/or Yellow Pages to make sure.
I should imagine that, with Spain falling within the Schengen Area as well, the application process would be more or less similar ... in that (thankfully, for once!) there's NO fee
for wives NOR dependent children of British Nationals.
Basically, the documents you WILL need for the application are:
Yours and Adam's passports
Your child(ren)'s passports(s)
Your original marriage certificate, and
Your child(ren)'s birth certificate(s)
I don't [think] proof of earnings are required (especially when you're only going on holiday) ... certainly WE weren't asked for this. But then, we did mention the fact that I'm an OAP and Myrna isn't in paid employment.