Hope you have better success than I did assembling a Sindy House in the loft in readiness for Christmas one year. When I went to get it out on Christmas eve it wouldn't fit through the hole ..... had to take it to pieces and re-assemble it .... 4AM to bed, kids up at 5 .... not a happy bunny!
What bike is it? Here's an assembley video and there are many more on youtube. Good like putting a bike together is as easy as falling off a bike (pardon the pun)
No gears (I'd have no chance) - don't know if i should risk it or wait for my husband (not that I have bought any bikes yet). (PS - and I will have to do two, Halfords says if I buy them today, they won't be ready until Monday and 3 year olds don't get the concept of waiting!)