I was told shortly before dd was 2 that I could have applied for a blue badge because she was using O2 cylinders. She's now 2.4 and uses a ventilator when asleep or tired/ cranky as well as a saturation monitor and on the occasion she isn't well will have an o2 cylinder in the mix as well . Is she still eligible for one? She has joint hypomobility and suspected Spd she still uses a pushchair continuously because she gets too tired walking with her resp condition but has no real mobility issues.
Higher rate dla mobility applies if you don't have it already for dc with serious respiratory support needs like oxygen, ventilator etc. Ignore the "must be 3" stuff altogether.
The criteria for mobility for little dc is needs very easy access to own vehicle for emergency medical equipment/ place to rapidly sort out any life-threatening complications / rush to hospital. Appeal if you get turned down, it will just mean the form got processed by someone inexperienced who thought 'computer said no'.