WOW - after reading your profile, I can say you need these spa dates even more than the rest of us.
I would suggest you spread them out at least 3-4 weeks apart. Given your need for ME time, I would suggest a massage. It's fine to tell them you have never had one and then go for a simple 30-45 minute massage. You need at least that amount of time to really relax. You can request a male or female massage therapist, whatever you want. They really work at protecting your dignity. You'll undress (underpants okay) and get under a sheet. They'll probably use a heated blanket for you, and put on some really relaxing music. Try to put your troubles behind you and just breathe deeply and relax. The therapist may chat with you for a moment, but otherwise there isn't a lot of talking - you shouldn't have to concentrate on too much conversation, you know? But it's great to have the tension worked out of your neck, back, arms, legs, even your facial muscles.
You could also opt for a facial, as someone else suggested - that may include a scalp massage, or may not. It usually includes a shoulder & arm massage. Sometimes you wear a strapless top they give you (and keep your pants on), other times they give you a longer strapless smock so you aren't in any constraining clothes and can relax. Your bottom half would be covered by a sheet though.
A pedicure can include a foot massage and that's really nice too.
I would use one certificate, and then wait a few weeks to use the other. I wouldn't get 2 facials in a row - you don't need them that close together. So if you are uncomfortable, get a facial & pedicure at the first spa (depending on the amount of the certificate) and then get a massage at the 2nd. That way you can ease yourself into the situation.
I wouldn't "waste it" on a manicure - go for something you cannot give yourself!
Get a babysitter for a longer period of time than you think you need so you don't have to rush right out and go home to kids & chaos. Maybe take yourself and a good book to lunch or meet a friend for a glass of wine?
And most of all, ENJOY IT!