Hi Snoops, having actually DONE what you are proposing, may I highly reccommend it? I would choose Lanzarote myself, again having actually DONE it.
It jsut wouldnt be a holiday if i didnt have my kids with me. To my mind, you have to get used to having your kids with you on hols, and you might as well get stuck in early!! My main concern would be the 10yr old, as to be hones, you may as well look after a 4 month old at home or in spain or in timbukto, the baby wont know the difference so long as you are prepared. I would check about the kids clubs, i think falcon do them all year round, althoug i could be mistaken.
Bring your steriliser if you can, bring enough formula for two weeks, bring enough nappies for two days, you can buy them over, bring some ready mad cartons of fomula for delays, emergencies etc, save some for the journey home, you never know! I thinks its a great age to bring an infant, jsut mind the sun, but theyw ill sleep great hopefully,a nd you can chill with a coffe in a bar, with baby fast asleep. How bad coudl it be?????? Best of luck HTH