Bits of each pattern, with her hand measurements thrown in for good measure…I think they will make both Peanut AND Mommy happy! Here we go!

Modifying as I go…

Pattern-ish Notes:
Cast on 40, twisted German, join to work in the round.
2x2 rib for 32 rows. (8P, 4W, repeat for rib)
Switch to stockinette stitch, Inc 2 on first round. (42)

Knit 4 rows before starting thumb gusset.
Inc for thumb as follows:
For left mitt:
K18, K1, M1, K1, M1, K around on odd rows.
Knit even on even rows.
(For right mitt:
K21, K1, M1, K1, M1, K around on odd rows.
Knit even on even rows.)
Repeat these two rows, increasing at edges of thumb (as established) until there are 11 thumb stitches. (14 rounds, I think?)
On next round, place thumb sts on holder, cast on 1 st in gap, and continue in st st until round 20 is finished.

If you want a flap, before knitting round 21, place waste yarn in the 21 stitches that will be the back of your mitten hand (or purl these stitches). Then continue knitting, being careful to not catch the waste yarn in your stitches.

Flap or no, continue knitting until round 28 is finished.

Start fingers -(mine worked in W)
On rnd 29 , place non-pinky sts on holder.

Work pinky finger over 10+2sts…6 rounds plain, 4 rounds 1x1 rib, stretchy bind off.

Ring finger is 4+5 sts from holders, 3 from pinky, 2 in gap (total of 14sts)…10 rounds plain, 4 rounds 1x1 rib, stretchy bind off.

Middle fnger is 5+5 sts from holders, 3 from ring finger, 2 in gap (15)…14 rounds plain, 4 rounds 1x1 rib, stretchy bind off.

Index finger is remaining 7+6+2 in the gap (15)…10 rounds plain, 4 rounds 1x1 rib, stretchy bind off.

Thumb - 11+5, knit for 18 rnds, dec & close thumb.

Mitten Flap
Cast on 22, then pick up the 21 stitches on the back of the hand. Join and, working in the round, knit 12 rounds 2x2 rib over cast on sts, knit plain on back of mitten. Switch to stockinette stitch for 18 more rounds (decreasing 1 stitch on first round - 42 total sts), then start decreases…

(K5, k2tog) around. (36)
K plain.
(K4, k2tog) around. (30)
K plain.
Continue decreasing, as established, until there are 12sts left. Close top of mitten by drawing tail through all stitches, or with kitchener stitch.