In 99.9% of cases, the sweat isn't causing the odour (the other 0.1% refers to ultra-rare metabolic disorders).
Fungal growth on the skin, which doesn't wash away in the shower, emits a nasty niff when it comes into contact with perspiration. Within a few hours, the BO smell can become really strong. It's becoming a more common problem since we all started washing at lower temperatures, doing mixed loads, and also since laundry liquid became more popular. The lower temperatures don't kill the bacteria and fungi in the wash load and liquids don't contain the bleaches that powders do. Our mums and grans used to wash at 60 degrees and do a weekly boil wash, killing any mould in the machine, so never had this problem. So, if there's even a little mould hiding at the back of the machine drawer; or, if one person in the house has athlete's foot, or has left something damp in the wash hamper (or a damp sports kit bagged up for a week) and it gets bunged in with everything else on a 30 degree cycle, it results in a nice warm bacterial/fungal soup for the clothes to slop around in. They remain in clothes and will happily colonise the rest of your jumper drawer in no time. Fungal spores are opportunistic little boogers and underarms are the perfect place for them to breed...
Best advice? For your dd, try an antifungal body wash, cream or powder. Artnaturals do a good tea tree body wash on Amazon, £13.95 for half a litre. A visit to the GP would be needed for the cream/powder. There are good OTC creams from the chemist, but as this is for a child (and I'm new here) I'm not going to recommend anything.
If your washer sounds the likely source of the problem, it can be remedied easily. A washing machine treatment can be bought from the supermarket, but run it at 90 degrees and ensure you don't just clean the powder drawer, but also the compartment it slots into. Running a 90 deg wash once a month will help maintain a clean machine. Halo Sports Wash (approx £6.80/litre) is an excellent liquid detergent containing antibacterials and antifungals, can be used just for sportswear and socks or add 30ml to your normal detergent. I prefer Goldsheild Laundry Additive (approx £23/litre). Although pricey, it provides antifungal and antibacterial protection for up to ten washes, to NHS standard. Only takes a small squeeze in the fabric softener drawer, plus it's ten-wash protection means you don't to need add it to every load. So a litre lasts AGES. Essential if there's a poorly person in your household.
I'm an ex-nurse, retired due to disability, so advice comes from a (fairly) informed basis 
. Hope this helps and good luck 
